Yes, they take some time to make, but they’re not hard to cook, and it’s so so worth the effort. Here is great way to enjoy them—in a simple rustic tart, with some shredded Gruyere cheese and a sprinkling of thyme. You can either use a ready made frozen crust (the flat or folded kind), or make your own crust. To prepare the dough, whisk the flour and salt together in a large bowl. Cut the butter into the flour with your hands or with a pastry cutter, until the mixture resembles a coarse meal. Add the sour cream and use a fork to incorporate it into the dough. Gather the dough into a ball and shape it into a disk. Wrap it with plastic wrap and chill for at least an hour. Cook, stirring occasionally for 10 minutes, until the onions have softened and are translucent. Reduce the heat to medium low and cook for an additional 40 minutes, stirring occasionally, until the onions are well browned. Add balsamic vinegar and cook for 10 minutes more, until onions are completely caramelized. Remove from heat. Roll out the dough on a lightly floured surface to a 13-inch diameter. Transfer the crust dough to a cookie sheet lined with parchment paper. Remove from oven and let sit for 10 minutes before serving.
