Caramelized Onion Tartlets without rolls

1 1/3 cups flour 1/2 teaspoon salt 8 tablespoons unsalted butter (chilled & cut into pieces) 3 -4 tablespoons cold water 1 Put flour and salt in the work bowl[...]

2 teaspoons olive oil 2 small red onions , thinly sliced 2 small yellow onions , thinly sliced 1 tablespoon white balsamic vinegar 1 sheet frozen puff pastry[...]