Makes: 8 servings. Prep: 20 minutes. Refrigerate: 30 minutes. bake: at 450 degrees F for 12 to 15 minutes; at 400 degrees F for 30 minutes.
Combine flour, egg yolk and salt in a large bowl. Quickly mix in butter with your hands until evenly blended. Add cold water by the tablespoon until you can gather the dough into a ball, about 2 to 3 tablespoons. Refrigerate 30 minutes or longer (up to 1 day).
Heat oven to 450°F. Coat a 10- to 12-inch pizza or tart pan with oil or butter. Roll out dough on a lightly floured surface, or press into pan up to edges. Bake at 450°F for 12 to 15 minutes, or until it just begins to color. Remove and cool on a rack (can be made an hour or two ahead).
Reduce temperature to 400°F. Spread crust with mustard. Core tomatoes; if very juicy, slice in half through the equator and squeeze out some of the juice, shaking out the seeds as well. Cut into 1/4-inch-thick slices. Layer onto crust, slightly overlapping. Season with thyme, salt and pepper; drizzle with olive oil.
Bake at 400°F for 30 minutes or until tomatoes are hot and crust is browned.