Adapted from: Simply Recipes
While it does take awhile to caramelize the onions, you don't need to stand over the stove stirring them. You can walk away and just give the onions an occasional stir while you go about doing other things. Once the onions are caramelized, this is a very quick dish to throw together. Just pie crust, cheese, onions, fold up and bake. It looks great when it comes out of the oven, too (see the photo on the original blog). Great vegetarian dish, too!
1 flat pie crust dough (pre-packaged or make your own)
3 medium sized red onions (use 4-5 if your onions are small)
2 Tbsp olive oil
1 Tbsp butter
1 teaspoon balsamic vinegar
1 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.
3 Heat olive oil and butter in a large, heavy-bottomed sauté pan or Dutch oven 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 center 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.