1 Pâte Brisée (tart dough) for one 10-inch tart (see method for making pâte brisée) or 1 packaged, flat pie crust (Trader Joe’s has one in their frozen section)
3 medium sized onions
2 Tbsp olive oil
1 Tbsp butter
1 teaspoon balsamic vinegar
3/4 cup (not packed) roughly grated Gruyère Swiss cheese
1 If you are making a crust from scratch, prepare the dough and let it chill in the refrigerator while you are cooking the onions.
2 Peel and slice the onions, lengthwise from stem to root.
3 Heat olive oil and butter in a large, heavy-bottomed sauté pan on medium heat. Once the butter has melted, add the onions and sprinkle a little salt over them. 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.
4 Preheat the oven to 450°F. Remove the dough from the refrigerator and let stand for 5 minutes before rolling it out. Roll out the dough on a lightly floured surface to a 10-inch diameter. Remove the crust dough to a cookie sheet lined with parchment paper.
5 Place all but a couple tablespoons of the cheese in the center of the dough. Spread to within 1 1/2 inches from the edges. Add the caramelized onions, layering them on top of the cheese. Fold the edges of the crust dough over so that a small circle of onion is still showing in the centre of the tart. Sprinkle the remaining cheese over the top of the tart.
6 Place in the oven on the middle rack. Bake for 10 minutes at 450°F. Reduce the heat to 350°F and bake for an additional 20-25 minutes, until the crust is golden brown. Remove from oven and let sit for 10 minutes before serving.