handful (about 15 or so) fresh thyme sprigs, or as much as needed to top
Instructions
First, make your caramelized onions. Heat olive oil in a cast-iron skillet over low heat. Spread thinly sliced red onions in a single layer in the pan and let cook slowly, stirring only occasionally, for at least 30-40 minutes and up to an hour. When onions are brown and jammy, add the balsamic vinegar and cook a little longer. Remove from the pan and set aside.
Roll out the dough of your choice to about 1/8-inch in thickness. Use a biscuit or cookie cutter to cut rounds to your desired size. Using a smaller cookie cutter or a knife, score a circle inside the pastry without cutting all the way through, then prick holes in the center circle with a toothpick or fork.
Top each round with a teaspoon of Boursin and a few slices of caramelized onions. If you’re using croissant dough, let the rounds sit and rise for about two hours. If you’re using puff pastry, go ahead and preheat your oven to 425 degrees. Using a pastry brush, brush the edges of the rounds with beaten egg, then bake at 425 for about 15 minutes, or until golden.
Optional: Make a balsamic glaze by combining balsamic vinegar and brown sugar in a small saucepan over medium heat. When mixture simmers, turn the heat down to medium-low or low and let bubble until reduced by half, about 10 minutes. Remove from heat and let cool briefly.
When the tarts are golden and puffed, top with small sprigs of fresh thyme and a drizzle of the glaze, and serve warm. Enjoy!