Exodus 23:19

The first of the choice fruits of thy ground thou shalt bring to the sanctuary of the Lord thy God. My people of the house of Israel, you are not permitted to dress or to eat of flesh and milk mingled together, lest I be greatly displeased; and I prepare you the wheat and the straw together for your food.

the first: ... And [Rashi] said that gedi (kid) is a young tender animal, and that this noun is [also] said for a young ox or sheep. And his proof is (in Genesis 38:20, where it states), gedi of the goats - for what [else] is the need for its qualification? But it is not like this, as only something from the species of goats is called a gedi. And in Arabic, it is [also] gedi, and it is not said for another species [besides goats]. However there is a difference between gedi and gedi of the goats; as the gedi is bigger than the gedi of the goats, such that [the latter] still needs to be with the [adult] goats. Hence, gedi and gedi of the goats. And the Sages received a tradition that Jews not eat meat with milk (as well as not to cook meat with milk). And now I will explain: You should know that the custom of the Torah is to speak about what is common. [For example with,] "the daughter of an ostrich" (Leviticus 11:16), why did Scripture alter it to say it like this? And does the daughter encompass the whole species? And we have not found it like this [with other species]. You should know that ostrich meat is dry like wood; and [so] it is not customary for people to eat it, as it has no moisture in it. And the only thing that is eaten from all of the species is the daughter - since it is female and young, it has a bit of moisture. But the young male does not. Likewise, people do not eat meat with milk, as it is not a pleasant food. As meat only gets cooked after a long time, and milk is not [quick to cook]; such that it is not the custom in the lands of Yishmael to this day for people to eat veal cooked in milk. [This is] because there is much moisture in veal, like there is [also] in milk, and so it is injurious - hence it is not eaten. But because gedi meat does not have moisture and it is warm when it is young, they therefore cook it in milk. And do not wonder, on account of it not being the custom of the people in these places to eat the gedi of the goats. For the physicians concur that there is no meat [as good] as it, and they even permit sick people to eat it. And in Spain, the Land of Israel, Persia and Babylonia, they do eat it. This was also the custom of the ancient ones (to eat it): "Two good gedii of the goats" (Genesis 27:9); "and we shall make a gedi of the goats before you" (Judges 13:15); and the priests eat a gedi of the goats as a sin-offering. And since no one eats raw meat, it is forbidden to cook a gedi in its mother's milk.... And we do not have a need to seek the reason for its prohibition, as it is hidden from the eyes of the ones with understanding. Perhaps it was because it is cruel-hearted to cook a kid in its mother's milk, in the way of "Do not slaughter a sheep or an ox - it and its child on one day (Leviticus 22:28), and "do not take the mother upon the young" (Deuteronomy 22:6). And most people are not accustomed to having sheep, so they buy milk from the marketplace and the milk is from a collection of different sheep. [Hence] maybe the buyer of the gedi will not know where is its mother; and when he buys milk, maybe there is milk there that is from the mother of the gedi that he bought, so he would transgress. And [we are] stringent about any doubt [concerning a law] from the Torah. And behold even the Yishmaelites concur that if a gedi is cooked in its mother's milk, such that the lineage is the same, then [the meat] is more pleasant. And the verse is speaking about what is common. And behold the ancient ones, may their memory be blessed, were stringent to remove any doubt and they forbade [all] meat and milk. And the Lord that gave them wisdom should give them their full wage....

ראשית בכורי אדמתך, this includes the seven types of products (Deut. 8,8) considered as the land of Israel’s choicest as per Deuteronomy 26,2 where you are commanded to bring the respective first ripe specimen of these seven products to Jerusalem as a gift. Ascent to Jerusalem with these products is to be on foot.

לא תבשל גדי בחלב אמו, it is usual for goats to give birth to two kids at the same time, and it was customary for people to slaughter one of them right away. Seeing that mother goats have an abundance of milk (expecting to nurse at least two young kids) they used the excess milk to boil the young kid in after it had been slaughtered. (Proverbs 27,27 “the goat’s milk will suffice for your food, the food of your household and the maintenance of your maids.”) It is something distasteful, revolting, something akin to gluttony, to consume the mother’s milk together with the young animal that this milk was intended to nourish. We find a parallel in the legislation not to slaughter mother animal and her young on the same day, as well as the prohibition not to take the young chicks while the mother bird is present. (Leviticus 22,25 and Deuteronomy 22,6-7) The Torah teaches you these matters as a matter of elementary culture, i.e. respecting life. Seeing that on the festivals many animals are consumed, the Torah included this legislation in the paragraph dealing with other aspects of these festivals. If the consuming of these animals is prohibited under such circumstances, how much more so are mixture of milk and meat prohibited as discussed in Chulin 113.

לא תבשל גדי בחלב אמו, do not practice these procedures which the idolaters believe are apt to improve the earth’s productivity. But, on the contrary, ראשית בכורי אדמתך תביא, if you really want to attract G’d’s blessing on your agricultural endeavours you will present G’d with a gift of the very first and finest of your respective crops. (compare Ezekiel 44,30 וראשית בכורי כל, וכל תרומת כל,...להניח ברכה אל ביתך, “all the choice first fruits of every kind, and all the gifts of every kind,- of all your contributions,- shall go to the priests,,,,that a blessing may rest upon your home.”