וּבִשַּׁלְתָּ֙ וְאָ֣כַלְתָּ֔ בַּמָּק֕וֹם אֲשֶׁ֥ר יִבְחַ֛ר יְהוָ֥ה אֱלֹהֶ֖יךָ בּ֑וֹ וּפָנִ֣יתָ בַבֹּ֔קֶר וְהָלַכְתָּ֖ לְאֹהָלֶֽיךָ׃
And thou shalt roast and eat it in the place which the LORD thy God shall choose; and thou shalt turn in the morning, and go unto thy tents.
Rashi on Deuteronomy
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).
Or HaChaim on Deuteronomy
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.
Rashbam on Deuteronomy
ובשלת, not in water but by roasting on a spit. (compare Rashi)