Preparation Arrange tomato slices on rack. Sprinkle with salt. Let stand 30 minutes. Meanwhile, heat oil in heavy large skillet over medium-high heat. Add onions and sauté until very soft, about 15 minutes. Add vinegar and thyme. Sauté until liquid is absorbed, about 2 minutes. Remove from heat. Preheat oven to 375°F. Line 2 baking sheets with parchment paper. Place bread dough on lightly floured work surface. Knead cheese and red pepper into dough. Divide dough into 8 equal pieces. Form each piece into ball. Roll out each ball to 3 1/2-inch round. Arrange on prepared baking sheets, spacing evenly. Using spoon, make depression in center of each round. Spoon onion mixture into each depression. Top each with 1 tomato slice; press gently to secure. Bake until breads are golden, about 30 minutes. Transfer to rack and cool completely.
Preparation Arrange tomato slices on rack. Sprinkle with salt. Let stand 30 minutes. Meanwhile, heat oil in heavy large skillet over medium-high heat. Add onions and sauté until very soft, about 15 minutes. Add vinegar and thyme. Sauté until liquid is absorbed, about 2 minutes. Remove from heat. Preheat oven to 375°F. Line 2 baking sheets with parchment paper. Place bread dough on lightly floured work surface. Knead cheese and red pepper into dough. Divide dough into 8 equal pieces. Form each piece into ball. Roll out each ball to 3 1/2-inch round. Arrange on prepared baking sheets, spacing evenly. Using spoon, make depression in center of each round. Spoon onion mixture into each depression. Top each with 1 tomato slice; press gently to secure. Bake until breads are golden, about 30 minutes. Transfer to rack and cool completely.