1. Roll out the dough 1/8 inch thick. Fit into tart pan, trim, and chill 30 minutes while preparing the onions and preheating the oven.

2. Preheat the oven to 425F with racks in the bottom third and upper third of the oven. If you have a pizza stone, place it on the bottom rack.

3. Set a large, heavy pot oven medium heat. Add oil and butter. When butter is melted, add onions and salt. Cook stirring occasionally until onions are tender, about 12 minutes. Turn down the heat to medium-low and cook stirring occasionally until onions are medium brown, about 45 minutes. Add balsamic vinegar (optional), and cook stirring occasionally until onions are dark nutty brown, about 15 minutes. Take off heat and set aside.

4. Place the tart pan on a cookie sheet. Line the tart shell with parchment paper or foil and dry beans (or some other weigh) and bake in the bottom third of the oven for 18 minutes. Remove parchment paper with beans, lightly poke dough with a fork at 1/4 inch intervals to prevent it from puffing up, and return to the bottom third of the oven for 5 more minutes.

5. Turn down the oven to 375F.

6. Fill the tart shell with caramelized onions. Sprinkle with a layer of gruyère. Arrange cherry tomatoes cut side down in concentric circles starting with the outer circle and working your way in towards the center of the tart. Bake tart on the bottom rack for 15 minutes. Move tart to the top rack and bake until tomatoes are just starting to brown, 15 more minutes. Cool 10 minutes before serving.

http://beyondsalmon.blogspot.com/2006/02/tomato-onion-tart.html