Step 1

Preheat the oven to 375 degrees F (190 degrees C).

Step 2

Heat butter in a skillet over medium heat and stir in onion. Pour in red wine to cover the bottom of the pan and sprinkle with sugar. Cook and stir until onions are tender and translucent, 10 to 15 minutes.

Step 3

Combine heavy cream, mozzarella cheese, cayenne pepper, salt, and black pepper in a bowl. Stir into onion mixture.

Step 4

Cut pie crusts into 3-inch circles using a cookie cutter. Place circles into the bottom of 12 muffin cups in a muffin tin.

Step 5

Spoon onion mixture evenly on top of the tart crusts.

Step 6

Bake in the preheated oven until a toothpick inserted into the centers comes out clean, about 20 minutes. Let cool 10 minutes before popping tartlets out of the muffin tin.