from https://ofbaking.wordpress.com/2011/10/03/ridiculously-good-caramelized-onion-tart/

ingredients

Phenomenal Tart Crust

• 1 ½ cup whole wheat stone ground flour
• 3 tbsp dark muscavado sugar
• 1 stick cold butter, cubed
• 1 tsp salt
• 1 egg yolk
• 3-4 tsp ice water
• (see frangipane tart crust recipe)

Caramelized Onion Filling

• ½ c mozzarella
• 2 tbsp romano
• 2 extra large onions
• ¼ cup mozzarella
• 4 cloves garlic, minced

directions

1. 1. Prepare pastry crust. Combine flour and sugar. Crumble between you fingers until sugar is fully incorporated. Add salt. Use a whisk to fully incorporate and aerate. Add butter. Using a pastry blender or your fingertips (rubbing butter into the flour), mix until the mixture resembles coarse bread crumbs. Add egg yolk. Incorporate fully. Slowly add just enough water so that the dough sticks together. Form into a ball and refrigerate for at least one hour.
2. 2. Cut onions into 1/4″ rounds. Place in a cold heavy-bottomed pan with just enough olive oil to coat the bottom of the pan. Toss onions and set on medium heat. Cook, stirring occasionally until onions begin to soften. Then, salt lightly, set heat to low, and cover. Cook, stirring occasionally until onions begin to color (maybe 20-30 minutes). Remove cover, add garlic, set heat to medium, and stir often until browned. Set aside.
3. 3. Remove pastry dough from the refrigerator and roll out to fit your tart pan. Gently lay into pan, careful not to press in, but gently set in. Fork the bottom and bake at 450 until lightly browned (10-15 minutes).
4. 4. Remove from oven and let crust cool. Spread 1/2 cup mozzarella cheese and 2 tbsp romano evenly on the tart crust. Then spread with onions, top with 1/8 cup mozzarella cheese and 2 tbsp romano.
5. 5. Cover with tin foil and bake at 450 for 30-45 minutes or until cheese is lightly browned.
6. 6. Sit down, slice off a piece, and enjoy.