A free-form tart (also known as a galette) is a budget-friendly way to dress up a meal with staple pantry ingredients. This recipe calls for rye flour and fresh rosemary, which take this tart firmly into cold weather territory with their respective nutty and piney overtones, but those can both be replaced with substitutions or omitted with equally stellar results. If you do stick with the recipe, don’t be tempted to overdo the rosemary- it’s meant to be an accent here. Add too much and you’ll miss the flavor nuances from the rye flour in the crust and some of the savory caramel notes in the onions. This tart is good hot, but it’s best at room temperature. Your patience will be rewarded.
Caramelized Onion Tart with Rosemary and Rye Crust
For the caramelized onions
3 1/2- 4 lbs Spanish onions, sliced fairly thinly
2-3 tablespoons olive oil or unsalted butter
For the tart shell
1 1/2 cups all-purpose flour
1/2 cup rye flour (if you don’t have this, you can substitute another flour, like whole wheat, or just use all-purpose flour)
3/4 teaspoon kosher salt
12 tablespoons cold unsalted butter, cut into smallish cubes
About 1/2 cup water, iced (measure the water before adding the ice)
1 teaspoon very finely chopped fresh rosemary (please don’t use the dry stuff)
1 egg + 1 tablespoon water, thoroughly beat together (for the egg wash)
1. Place the onions, olive oil, and a few healthy pinches of kosher salt into a large skillet and turn the heat up to medium. Once the onions start to sizzle, turn the heat down to medium low/low and maintain the heat low enough that the onions won’t brown. Cook the onions this way, stirring occasionally, for about an hour, or until they are squidgy, sweet, and golden. Don’t take it all the way to a deep caramel color, though- they’ll get there once you’re baking them in the oven. Once the onions are done cooking, remove them from the heat and let them cool thoroughly.
2. While the onions cook, make the tart dough. Mix the flours and salt in a large bowl, then add the butter. Use a pastry cutter, a fork, or your fingers, work quickly to cut the butter into the flour until you have chunks of butter ranging in size from pea-sized to the size of small lima beans.
3. Drizzle about 2/3 of the water into the flour and, using a light but fast hand, use a fork to blend the water and flour together just until things start to clump together. Keep going until you only have a little dry flour left, drizzling a little water at a time. You want to avoid using too much water- it results in a tough dough. Use only enough to make things just cohesive. Turn the dough out onto a floured countertop, sprinkle a bit of flour on top, and flatten into a disc.
4. Wrap the disc in plastic wrap and refrigerate for an hour before using. You can make the dough up to three days in advance and keep it in the fridge, wrapped this way.
5. When you are ready to bake the tart, line a large baking sheet with parchment paper and set aside. Place the dough on a floured surface and roll it out into a rough circle, about 14″ in diameter. It’s ok if it ends up looking more like an amoeba than a circle- this is one pastry that doesn’t need clean edges. Gently lift the pastry from the counter and place it on the baking sheet (if you’re not sure how to do this, one way is to gently fold it in half, then in half again, then lift it from the counter and transfer it to the sheet, then unfold it) and sprinkle the rosemary over the surface of the pastry. Place the dough in the refrigerator for about 20 minutes. Don’t skip this step- you want the butter to be very cold when it goes into the oven, and rolling out the dough warmed it up a bit. Chill it.
6. While the dough is chilling, preheat your oven to 375 degrees. Once the dough is chilled and the oven has reached temperature, remove the baking sheet from the refrigerator and, using a spoon, spread the caramelized onions over the dough, leaving a 1 1/2-2″ perimeter untouched. Fold the perimeter over the onions, then use a pastry brush to brush the folded edge with egg wash.
7. Bake for 50-60 minutes or until golden brown. The underside of the crust should be browned, and the onions should deepen in color. Start checking around 35 minutes, just to be sure that everything is looking good. Remove from the oven and let cool to room temperature, then cut into wedges and serve.