Komentarz do Powtórzonego Prawa 16:7

ובשלת is identical with צלי אש “roast with fire” mentioned in Exodus 12:9, for it (roasting) also comes under the term בשול “cooking” (Mekhilta d'Rabbi Yishmael 12::9:2).

ובשלת ואכלת, "you shall cook it and eat it, etc." In this instance the Torah uses the term "cook" although in Exodus 12,9 "boiling" or "cooking" the passover has been expressly prohibited. Perhaps the Torah here refers back to the peace offering which accompanies the passover which may also consist of cattle, not only of sheep (compare 16,2) Our sages in Pessachim 70 read the words צאן ובקר in that verse as meaning that the passover has to be from the category of animal called צאן, whereas the peace-offering may consist of בקר, cattle. I have elaborated on this subject in my volume פרי תואר on the Yoreh Deyah.

ובשלת, not in water but by roasting on a spit. (compare Rashi)