From: Rabbi Moshe Sofer
To: Unknown recipient
The Chasam Sofer proposes that the Torah term bishul in basar bechalav encompasses any absorption in which fat permeates throughout the food, making roasting fatty meat with milk a Torah-level prohibition while roasting lean meat is only rabbinic. He engages with the Pelesi's proofs based on Rashi's interpretation, concluding that according to Rashi one does receive lashes for roasting in basar bechalav. He also clarifies that within a single piece the issur spreads immediately for purposes of bitul, but between separate pieces a time delay prevents combining them for nullification.
Source: Shu"t Chasam Sofer YD §97