How to Make It
Heat oil in a skillet over medium-high heat. Add onions; sauté 5 minutes. Reduce heat to medium. Cook for 15 minutes; stir occasionally. Add vinegar.
Preheat oven to 400°.
Unfold dough, and place on a work surface lightly dusted with flour. Roll into a 10 x 9-inch rectangle. Cut dough into 20 (2 1/4 x 2-inch) rectangles. Score each rectangle about 1/8-inch from edge. Prick dough pieces liberally with a fork. Arrange on a baking sheet lined with parchment paper; chill 10 minutes.
Sprinkle about 3/4 teaspoon cheese over each tartlet; top with about 1 tablespoon onion mixture. Sprinkle with salt. Bake at 400° for 15 minutes or until dough is lightly browned.