Commentary for Sanhedrin 8:31

It is the Way of Boiling the Torah Forbade And with fat it is not considered boiling, rather frying, like it says in Rashi. And that which we call pan roasted as boiling in the chapter of 'Kol Shaah' (Pesachim 41a) is different there, because there it says "and boiled" (Exodus 12:9) with anything (any form of liquid here, including fat, would be considered boiling). And there are those that say that the Torah prohibited boiling [in milk], because without boiling [in milk], [the food] would have been allowed; through boiling [in milk] it (the food) was prohibited - to exclude [the thought that we are referring to] forbidden fat, because [cooking in forbidden fat] is prohibited either way. This explanation is difficult because if you have forbidden fat and [meat from animal which] died, this is forbidden also with the prohibition of forbidden fat, as it says in the chapter of 'Kol Habasar' (Chulin 113b and 115a) according to one who says that multiple prohibitions can stack - and we say over there that forbidden fat and an animal which died doesn't need an extra verse to prohibit it.