Avodah Zarah 38 - Cooked Food of Idolaters
Cooked food of idolaters is prohibited even when it is cooked in kosher utensils and all its ingredients are kosher. Why? - Because the Jews sent a message to Sichon, King of the Amorites, " We will buy your food and eat, and buy your water and drink. " This means that they will only eat food that has not changed from its raw state, just like water. But if so, then parched grains made by idolaters should be forbidden, and we know that they are allowed?! - Rather, this Torah phrase serves only as a hint, and the prohibition is Rabbinical.
If a Jew takes part in the cooking processes, whether in the beginning, the middle, or the end, the food is no longer prohibited, even though the rest of the cooking was done by an idolater. While baking bread, even if a Jew throws one chip of wood into the fire, the bread is no longer prohibited as cooked by an idolater. Whether throwing in a chip of wood is sufficient for foods other than bread is a subject of disagreement.
Art: Bartolome Esteban Murillo - Baking of Flat Cakes