Parashas Bo · Shemos 12:9 · TT note 13
Torah Temimah on Shemos 12:9, note 13 (on כִּי אִם צְלִי אֵשׁ) points to Mechilta.
Sefaria — Exodus 12:9
Sefaria — Torah Temimah on Exodus 12:9
Sefaria — Mechilta
English: Why does it say “but”? One might think that what is fit for boiling may be boiled and what is fit for roasting may be roasted — therefore it says “but roasted with fire.”
Exclusionary derash: no option to boil; the only permitted preparation is fire-roasting.
One may not choose boiling for any part of the Pesach; the entire offering is fire-roasted.