According to Redyoshi's guide (pinned post in the community tab), on the Hell's Kitchen section, "Once the war ends and Thane is in the tavern, and you have cooked at least 5 of Parif’s dishes, talking to Thane will have him mention how he misses cooking, and wants to become Parif’s apprentice to cook and be useful to the tavern."

This guide may have spoilers, but it describes the game with perfect detail.