Exodus 23:19 The first of the firstfruits of thy land thou shalt bring into the house of the LORD thy God. Thou shalt not seethe a kid in his mother’s milk.
As John Gill writes you can see how far astray the Jews went on this law.
thou shalt not seethe a kid in his mother’s milk: and so a calf, or a lamb (f), as Jarchi interprets it; which some understand of slaying a young kid and its dam together, and so is a law against cruelty, like that law of not taking the dam with the young, on finding a bird’s nest, Deu 22:6 others, of killing, dressing, and eating a kid, while it sucks the milk of its mother, before it is eight days old, and so a law against luxury;
but the Jews generally understand it of boiling, or eating the flesh of any creature and milk together (g): so the Targum of Onkelos paraphrases it,”ye shall not eat flesh with milk;”and the Targum of Jonathan is,”ye shall neither boil nor eat the flesh and the milk mixed together:”hence, according to the rules they give, the flesh of any beast, or of a fowl, is not to be set upon a table on which cheese is (being made of milk), lest they should be eaten together; nor may cheese be eaten after flesh until some considerable time, and then, if there is any flesh sticks between a man’s teeth, he must remove it, and wash and cleanse his mouth; nor may cheese be eaten on a table cloth on which meat is, nor be cut with a knife that flesh is cut with (h): so careful are they of breaking this law, as they understand it:
but the words are, doubtless, to be taken literally, of not boiling a kid in its mother’s milk; and is thought by many to refer to some custom of this kind, either among the Israelites, which they had somewhere learnt, or among the idolatrous Heathens, and therefore cautioned against; Maimonides and Abarbinel both suppose it was an idolatrous rite, but are not able to produce an instance of it out of any writer of theirs or others: but Dr. Cudworth has produced a passage out of a Karaite author (i), who affirms,”it was a custom of the Heathens at the ingathering of their fruits to take a kid and seethe it in the milk of the dam, and then, in a magical way, go about and besprinkle all their trees, fields, gardens, and orchards, thinking by this means they should make them fructify, and bring forth fruit again more abundantly the next year:”and the Targum of Jonathan on Exo 34:26 seems to have respect to this, where, having paraphrased the words as here quoted above, adds,”lest I should destroy the fruit of your trees with the unripe grape, the shoots and leaves together:”and if this may be depended upon, the law comes in here very aptly, after the feast of ingathering, and the bringing in the first fruits of the land into the Lord’s house.
If you examine the whole chapter the dominant theme is about the false gods and worship and the one true God.
Exo 23:13 And in all things that I have said unto you be circumspect: and make no mention of the name of other gods, neither let it be heard out of thy mouth.
Exo 23:17 Three times in the year all thy males shall appear before the Lord GOD.
Exo 23:19 The first of the firstfruits of thy land thou shalt bring into the house of the LORD thy God. Thou shalt not seethe a kid in his mother’s milk.
Exo 23:24 Thou shalt not bow down to their gods, nor serve them, nor do after their works: but thou shalt utterly overthrow them, and quite break down their images.
Exo 23:25 And ye shall serve the LORD your God, and he shall bless thy bread, and thy water; and I will take sickness away from the midst of thee.
Exo 23:32 Thou shalt make no covenant with them, nor with their gods.
Exo 23:33 They shall not dwell in thy land, lest they make thee sin against me: for if thou serve their gods, it will surely be a snare unto thee.