Directions

1. On a lightly floured surface, roll out dough to a 12-inch circle. Fit dough into a 10-inch round fluted tart pan with a removable bottom, pressing into edges. Trim dough to a 1/2-inch overhang all around. Fold edge of dough over to create an edge that extends 1/4-inch above the pan. Lightly prick bottom of dough all over with a fork. Chill until firm about 30 minutes.
2. Preheat oven to 375 degrees. Line bottom of dough with parchment paper, leaving at least a 1-inch overhang. Fill with pie weights or dried beans. Bake until the edges are just beginning to turn golden, about 30 minutes. Remove parchment paper and weights; continue baking until lightly golden all over, 10 to 15 minutes more. Cool tart shell completely on a wire rack.
3. Heat olive oil in a medium skillet over medium heat. Add leeks and thyme and cook, stirring occasionally, until leeks soften and begin to brown, 10 to 15 minutes. Season with salt and pepper.
4. Sprinkle half of the cheese over the bottom of the tart shell, top with cooked leek mixture and sprinkle with remaining cheese. In a medium bowl, whisk together eggs, yolk, milk, cream, and a pinch of salt and pepper. Pour mixture over leeks and cheese. Bake until puffed and golden brown, 30 to 35 minutes. Transfer to a wire rack to cool 10 minutes before serving, or serve at room temperature.