Midrasz do Diwrej ha-jamim II 35:13

"uvashel": "bashel" (here refers to flesh that was) roasted (before, the understanding being that it is forbidden to cook it even if it had been roasted previously), as in (Devarim 16:7) "And you shall cook (i.e., roast) it and you shall eat it," and (II Chronicles 35:13) "And they cooked (i.e., roasted) the Pesach in fire as prescribed; and the holy (offerings) they cooked in pots, cauldrons, and pans, and they dispatched them to all the people." From here R.Yoshiyah said: If one takes a vow not to eat "mevushal," roasted (flesh) is (also) forbidden to him. "but roasted in fire": What is the intent of this? I might say: What is (more) fit for roasting should be roasted. It is, therefore, written "but (i.e., exclusively) roasted in fire,"