Melt 2 Tbsp. butter in a large, heavy skillet over low heat. Add onions, 1/2 tsp. salt and thyme; cook until tender and lightly browned, 30 minutes. Set aside to cool.
Place one rack in upper third of oven and another in lower third; preheat oven to 400°F. Coat 2 baking sheets with cooking spray.
Mix flour, baking powder, sugar and remaining 1/2 tsp. salt in a bowl. Cut remaining 6 Tbsp. butter into small chunks and add to flour mixture. Cut in butter using your fingers, 2 knives or a pastry blender until mixture resembles coarse meal. Stir in milk with a fork just until flour is evenly moistened.
Turn out dough onto a floured surface and pull together into one piece with floured hands. Divide dough into 48 mounds (about 1/2 Tbsp. each); roll each into a ball and place on baking sheets. With a teaspoon measuring spoon, press an indentation into center of each ball. Spoon onion mixture into indentations.
Bake, rotating pans halfway through, until golden, 12 to 15 minutes. Serve warm.
My five stars are based on how simple this is to make, and how many it can serve. I agree with the review that you can double the dough recipe and still have plenty of topping. I added a ricotta cheese base that made the onion tart spectacular. Mix one cup ricotta cheese, one egg, and one tsp powdered sugar(enough topping for double dough recipe). Spread a thin amount on the flattened tart and top with onion. Delicious and surprisingly impressive. Your guests will say something for sure.
The proportions of this recipe need to be worked on. I didn't have enough dough, and way too much onion. Also, the tarts never browned, and started to burn quickly. The rose too much, pushing the onion off of the top. More dough, bigger tarts, and they would rock.
Sign Up for our Newsletter
Join our newsletter for free recipes, healthy living inspiration, and special offers.