Directions

1. To make crust, combine flour, sugar, salt and thyme leaves in a food processor and pulse until blended. Add the butter and pulse 5 times. Next, add the egg and heavy cream, pulsing until dough begins to form. Remove dough from food processor and form into a ball. Lightly flatten dough, and wrap in plastic wrap. Refrigerate for at least 30 minutes. Dough can be made up to 1 day in advance.
2. Preheat oven to 375 degrees. In a small bowl, mix together cider vinegar and mustard. Slowly whisk in olive oil until incorporated. Season with salt and set aside.
3. Remove dough from refrigerator and allow to come to room temperature. Lightly dust a large sheet of parchment paper with flour. Place the dough on the surface and roll it out to a circle approximately 12 inches in diameter and ⅛ inch thick. Transfer dough to rimmed baking sheet. Starting 2 inches in from the edge, top the dough with Gruyère. Place apple slices in a circular pattern on top of cheese until you have reached the center. Fold the outer edges over the apples around the entire outside of galette. Lightly brush edges with heavy cream and bake for 30 minutes; remove from oven.