Mixing Meat And Milk

(יט) רֵאשִׁ֗ית בִּכּוּרֵי֙ אַדְמָ֣תְךָ֔ תָּבִ֕יא בֵּ֖ית יְהוָ֣ה אֱלֹהֶ֑יךָ לֹֽא־תְבַשֵּׁ֥ל גְּדִ֖י בַּחֲלֵ֥ב אִמּֽוֹ׃ (ס)

Exodus 23:19

(19) The choice first fruits of your soil you shall bring to the house of the LORD your God. You shall not boil a kid in its mother’s milk.

This text explains that you can´t cook a kid, specifically a goat in it´s mothers milk. This same phrase is shown twice more in the Torah.

חולין קט״ו ב:ו׳

דבי רבי ישמעאל תנא (שמות כג, יט) לא תבשל גדי בחלב אמו ג' פעמים אחד לאיסור אכילה ואחד לאיסור הנאה ואחד לאיסור בשול

Chullin 115b:6

The school of Rabbi Yishmael taught: The Torah states three times: “You shall not cook a kid in its mother’s milk” (Exodus 23:19, 34:26; Deuteronomy 14:21). One verse serves to teach the prohibition against eating meat cooked in milk, and one serves to teach the prohibition against deriving benefit from it, and one serves to teach the prohibition against cooking meat in milk.

This commentary explains that one time in the Torah the sentance is prohibiting eating meat and milk, once cooking it, and once gaining benefit from it.

חולין קט״ו א:י׳

אמר ריש לקיש מנין לבשר בחלב שאסור ת"ל (שמות יב, ט) אל תאכלו ממנו נא ובשל מבושל שאין תלמוד לומר מבושל מה תלמוד לומר מבושל לומר לך יש לך בשול אחר שהוא כזה ואי זה זה בשר בחלב

Chullin 115a:10

§ Reish Lakish says: From where is it derived that meat cooked in milk is prohibited for consumption? The verse states with regard to the Paschal offering: “You shall not eat it partially roasted, nor boiled in any way” (Exodus 12:9). As there is no need for the verse to state: “Boiled in any way,” since it could simply have stated: “Boiled.” What is the meaning when the verse states: “Boiled in any way?” It is included to tell you that there is another manner of cooking, the product of which is prohibited like this one. And which cooking is this? This is meat cooked in milk.

This says that the pasuk is not just referring to boiling with water, but also meat in milk. Reish Lakish actually gets this from a different phrase, so the Torah talks about meat and milk more than those three times.

יורה דעה פ״ז:ב׳

גדי לאו דוקא דהוא הדין שור שה ועז ולא שנא בחלב אם ולא שנא בחלב אחרת אלא שדבר הכתוב בהווה:

Shulchan Arukh, Yoreh De'ah 87:2

[The word] "kid", is not specific, it is the same rule with ox, sheep and goat. There is no difference if it is the milk of the mother, or of another [domesticated] animal. Rather, the verse refers to the common case.

According to Rabbi Yosef Karo, author of the Shulchan Aruch,the rule on meat and milk is talking about any domesticated animal, not just a goat. He was a Rishon.

משנה תורה, הלכות מאכלות אסורות ט׳:ג׳

אֵין אָסוּר מִן הַתּוֹרָה אֶלָּא בְּשַׂר בְּהֵמָה טְהוֹרָה בַּחֲלֵב בְּהֵמָה טְהוֹרָה שֶׁנֶּאֱמַר (שמות כג יט) (שמות לד כו) (דברים יד כא) "לֹא תְבַשֵּׁל גְּדִי בַּחֲלֵב אִמּוֹ". וּגְדִי הוּא כּוֹלֵל וְלַד הַשּׁוֹר וְלַד הַשֶּׂה וְלַד הָעֵז עַד שֶׁיִּפְרֹט וְיֹאמַר גְּדִי עִזִּים וְלֹא נֶאֱמַר גְּדִי בַּחֲלֵב אִמּוֹ אֶלָּא שֶׁדִּבֵּר הַכָּתוּב בַּהוֹוֶה. אֲבָל בְּשַׂר בְּהֵמָה טְהוֹרָה (שֶׁבִּשְּׁלוֹ) בַּחֲלֵב בְּהֵמָה טְמֵאָה. אוֹ בְּשַׂר בְּהֵמָה טְמֵאָה (שֶׁבִּשְּׁלוֹ) בַּחֲלֵב בְּהֵמָה טְהוֹרָה מֻתָּר לְבַשֵּׁל וּמֻתֶּרֶת בַּהֲנָיָה וְאֵין חַיָּבִין עַל אֲכִילָתוֹ מִשּׁוּם בָּשָׂר בְּחָלָב:

Mishneh Torah, Forbidden Foods 9:3

Biblically, only the meat of a clean animal with the milk of a clean animal is forbidden, as it is written: "You shall not boil a kid in its mother's milk." The term kid includes the offspring of ox, sheep or goat, unless the text explicitly states "a kid from the flock of goats" (Genesis 38:17). The phrase "a kid in its mother's milk" is used only because Scripture speaks of something that actually happens. But if one cooks the meat of a clean animal with the milk of an unclean animal, or the meat of an unclean animal with the milk of a clean animal, he is permitted to do so and derive a benefit from it as well; punishment is incurred for eating it, but not by reason of its being a meat-milk mixture.

This piece of the Mishneh Torah written by Rambam, a rabbi from the time of the Rishonim teaches that you can get benefit from meat and milk if one of the pieces (meat or milk), is impure. I was wondering what pure meant.

יורה דעה פ״ז:ג׳

אינו נוהג אלא בבשר בהמה טהורה בחלב בהמה טהורה אבל בשר טהורה בחלב טמאה או בשר טמאה בחלב טהורה מותרי' בבישול ובהנאה ובשר חיה ועוף אפילו בחלב טהורה מותר בבישול ובהנאה ואף באכילה אינו אסור אלא מדרבנן אבל דגים וחגבים אין בהם איסור אפילו מדרבנן: הגה ונהגו לעשות חלב משקדים ומניחים בה בשר עוף הואיל ואינו רק מדרבנן אבל בשר בהמה יש להניח אצל החלב שקדים משום מראית העין כמו שנתבאר לעיל סי' ס"ו לענין דם (ד"ע):

Shulchan Arukh, Yoreh De'ah 87:3

The law only applies with meat from a pure [kosher] animal and milk from a pure animal, but meat of a pure animal in milk that is impure, or meat from an impure animal in pure milk, is permitted to cook and benefit from. Meat of a wild animal and of fowl, even in milk which is pure, is permitted to cook and benefit from; and even eating is only prohibited rabbinically. Fish and grasshoppers are not prohibited [with milk] even rabbinically. We make milk from almonds and place bird meat in it, since [milk and bird meat] is only rabbinically [forbidden]. But with meat from a domesticated animal, place almonds next to the milk, so that people don't misunderstand. This is as we said above, in chapter 66.

Rabbi Yosef Karo preaches that pure means kosher, and you can benefit from a wild animal in milk. There are specific rules about non dairy milk having a Heker. The piece in italics comes from the Rem"a (in the Mapah), while the Rest is Rabbi Yosef Karo.

Using An Oven For Both Meat & Dairy

By Rabbi Dovid Heber

If a pan with meat and a pan with dairy are covered, one may place both of them in an oven l’chatchila. This is true, provided that the pans do not touch each other and do not touch the “opposite” food (e.g., a meat pan may not rest on a rack with dairy residue).
If only one pan is covered and is on the lower shelf, it is also muttar l’chatchila. An uncovered meat pan that contains liquid may not be placed below a dairy pan in the oven (even if the top pan is covered).
It is prohibited for one to bake milk and meat products uncovered at the same time in an oven. B’dieved, if one erred and baked dairy food and a meat product in the oven at the same time (in different pans), the following halachos apply:

If both the milk and meat dishes were uncovered, and they were not liquidy and not charif (sharp), everything is b’dieved kosher. If either the pan with meat or the pan with dairy is liquidy or sharp (and they are both uncovered), the oven and all the food in the oven may be treif and the keilim (vessels) used with these dishes may require kashering.
If two pans touch, the following applies: If a pan with meat touched a pan with dairy in the oven (and there was no residue on the outside of either one), the food and keilim are b’dieved kosher.
If a meat oven is clean, one may bake a dry, uncovered (or covered) dairy item in it. It is not necessary to first kasher the oven or wait 24 hours. If one wants to bake a dry, uncovered dairy item immediately after cooking meat, one should first wait for the oven (which must be clean) to cool down.

An Achron, Rabbi Dovid Heber from Star K, talks about cooking meat and milk in the same oven. He says that you need to cover the pans to block the smell from spreading. He gives real life examples of what to do in different situations.