Directions

1. Preheat the oven to 400 degrees F. Lightly flour a baking sheet and set aside.
2. Heat the oil in a large frying pan over medium-low heat. Add the onions and stir a few times, to distribute the oil evenly. Add the salt and pepper and stir again. Cook for 18-20 minutes, allowing to caramelize, stirring once or twice throughout cooking.
3. While the onions caramelize, divide the dough into 4 even balls on a lightly floured surface. Use your hands or a rolling pin to roll each ball out into a 6" circle. Set the circles on the prepared baking sheet (it's OK if they touch--they won't when you're done assembling the tartlets).
4. Arrange about half-an-apple's worth of slices on a circle (as if you were making apple pie) in the center of one of the dough circles. Make sure to leave about 1 1/2 inches of dough at the border.
5. Top the circle of apples with about 1/4 of the caramelized onions.
6. Fold the border of dough in, to form a tartlet (it's supposed to look rustic, so don't worry about making it look perfect). Sprinkle the whole thing (crust included) with about 1/8 cup cheddar cheese.
7. Repeat the tartlet-making process with the remaining ingredients.
8. Bake for 18-22 minutes, or until the crust is golden-brown and the cheese is bubbly.
9. Serve whole or cut into wedges.