This tart contains many of the same flavors as French onion soup, with lots of sweet caramelized onions and Gruyere cheese; in the pie, the croutons are replaced by a fantastic (and super easy) pie crust. This is absolutely one of the easiest pie crusts I've ever made, and I will definitely use it again. I baked it in a 10-inch springform pan so I could get the sides tall enough to hold all the filling; you could also use a 10-inch quiche pan if you have one. The onions take a while to caramelize, but they are so worth it. Add in cream, eggs, and cheese, and you have a recipe for French onion soup in pie form! The tart is very rich, so I'd recommend serving it with a side of veggies with a bit of acid (we had broccoli from the garden with lemon juice squeezed on top; a salad with vinegar-y dressing would be excellent as well).
French Onion Tart (adapted from Fine Cooking)
2 T vegetable oil
2 T unsalted butter
4 medium onions, very thinly sliced
1 tsp granulated sugar
Kosher salt and freshly ground black pepper
3 large eggs
1 1/4 cups heavy cream
A tiny pinch nutmeg (preferably freshly grated)
2/3 cup grated Gruyère
1 partially baked tart shell in a 10-inch springform pan (or a 10-inch quiche pan) (see below)
1. Heat the oil and butter in a large skillet. Sprinkle the sugar into the pan, and then add the onions. Sauté over medium heat, stirring constantly, until the onions start to soften. Lower the heat and continue to cook the onions until very soft and evenly browned, stirring frequently, another 30 to 40 minutes. The onions should be very soft and look almost "melted": if they start to get darkened or crispy, turn the heat down. When the onions are nicely browned and soft, transfer them to a strainer and drain the oil.
2. Heat the oven to 375°F. Place the springform pan on a baking sheet.
3. In a bowl, whisk together the eggs and cream. Whisk in 3/4 tsp salt, a scant 1/2 tsp pepper, and the nutmeg. Add the drained onions and half of the cheese and whisk until everything is well combined. Fill the prepared tart shell with the onions and custard. Top with the remaining cheese and bake until the tip of a knife comes out clean and the top of the tart is puffed and brown, 40 to 45 minutes Let cool for at least 15 to 20 minutes before serving.
Tart Dough (adapted from Fine Cooking)
6 3/4 oz. (1 1/2 cups) all-purpose unbleached flour
1/4 tsp plus a pinch of table salt
4 1/2 oz. (9 T) unsalted butter, cut into small pieces and chilled
4 T ice water
1. Combine the flour, salt, and butter in the bowl of a food processor. Using short pulses, process until the mixture resembles oatmeal. Add the ice water and pulse quickly until the mixture begins to form a ball. Remove the dough and gently flatten the ball into a smooth disk about 1-1/2 inches thick. Wrap it in plastic and refrigerate until firm enough to roll, at least 1 hour.
2. Roll the dough out on a lightly floured surface to a circle of about 14 inches in diameter. Transfer to a lightly greased springform pan and press into the bottom and sides. Cut around the top of the dough so that there is an even edge.
3. Prick the dough with a fork all over, and then freeze at least 30 minutes. Preheat the oven to 425 degrees.
4. Line the pie shell with foil or parchment and fill with dried beans or pie weights. Bake for 12 minutes, then remove the weights and foil and bake an additional 6 minutes. Cool until needed.