פירוש על עבודה זרה 75:12

The Talmud notes that the statement by Shmuel seems to contradict a ruling by R. Yohanan. R. Yohanan said that if the food had already been cooked a minimal amount (sufficient for the bandit Ben Drosai to eat it—we will discuss this more later) then the prohibition of being cooked by a non-Jew does not apply. But if the food was not yet cooked, then if the non-Jew does turn it over on the coals then the food is prohibited. This contradicts what Shmuel said before.
The resolution is that R. Yohanan was referring to a case where the non-Jew essentially began the cooking process by putting the pot in the oven. The Jew only put the food into the pot. But if the cooking process had already begun, then the prohibition of non-Jewish cooking does not apply. This seems to be a considerable leniency.