Because of the halacha of Bishul Akum (or Bishul Yisrael, call it what you will), even if a food's ingredients are all kosher, it needs to have a Jew involved in the cooking process for it to be kosher. There are many details here, and some forms of cooking are different than others. But what about a flambé: if a non-Jew ignites it, does that make the food non-kosher?
If the food is already cooked (to the level of Maachal ben Drusai), and the flambé is just adding some flavor, it would not be problematic (Y.D. 113:8)