Ta'am basar b'chalav (part II)

Yesterday's post touched on the idea that basar b'chalav that is less than a shiur is not assur because "derech bishul asra torah" and one does not cook a drop (see Tosfos Avodah Zarah 65b). Tosofs im A"Z does not quote the gemara in Chulin 108, but it seems to me that is their source. The Mishna in Chulin says that if there is no ta'am, there is no issur of basar b'chalav. Abaye extrapolates from basar b'chalav to all issurim and says ta'am is a din d'oraysa everywhere, because otherwise, if it is a special din chiddush only in basar b'chalav, why is there a shiur that depends on taste? Rava argues and holds that even if there is not a halacha of ta'am in kol hatorah kula, basar b'chalav is still assur only if there is a shiur of ta'am because "derech bishul asra torah". In other words, you need ta'am to be considered cooking; if there is no ta'am, then it is not derech bishul.
So why can't I cook a drop of milk in my cholent - it's not derech bishul?!

(My wife thought of a joke to insert here. The couple was invited to the non-religious in laws for dinner, which led to kashrus concerns for the newly religious couple. "No problem", said the husband, "Your mother's cooking has no ta'am anyway.")