A rich, herbaceous caramelized onion tart that’s perfect for a show-stopping holiday app or side. If you’re short in time, swap in store bought all-butter puff pastry dough.
- 1 1/4 cups all-purpose flour
- 1 teaspoon sugar
- 1/2 teaspoon sea salt
- 8 tablespoons unsalted butter, cut 1/2-inch cubes
- 1/3 cup ice water
- 4 medium yellow onions, halved lengthwise and peeled
- 2 tablespoons unsalted butter
- Salt
- Black pepper
- 2 bay leaves, preferably fresh
- 4 thyme sprigs, plus more for garnish
- 1 tablespoon ground Dijon mustard
- 1/2 cup grated Gruyere
- 1/2 cup grated Parmesan
- 1 egg, lightly beaten
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Working 1 hour or up to 3 days ahead, make and chill the dough. In a large bowl, combine flour, sugar, and salt. Fold in butter. Working quickly, use fingertips to smush and flatten most of the butter cubes, ensuring each piece is completely coated with flour. You should have a shaggy, clumpy dough with some large butter chunks when you're done. Fold in ice water, gently nudging dough until it holds together. If needed, add 1 - 2 tablespoons more ice water to help dough hold together.
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Turn dough out onto a sheet of parchment or plastic wrap, shape into a disc, and wrap tightly.
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Chill at least 1 hour or up to 3 days ahead.
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Use a tiny triangle cut to remove onion cores. Then cut 1/4-inch thick slices lengthwise (from top to root).
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Set a wide, deep-sided 10-inch skillet or pot over medium heat. Once hot, add the butter, and then the onions a few handfuls at a time. Set timer for 10 minutes and cook, stirring frequently until the onions have taken on color and just started to sizzle. Reduce heat to medium-low, add bay leaves, and a 1/4 teaspoon each salt and pepper. Set timer for 40 minutes. Check the onions frequently, stirring and turning them. If the skillet starts to dry out, add a tablespoon of water and be sure to scrape up the delicious brown bits from the bottom of a skillet. Toward the end of cook time, add additional salt and pepper to taste. Cook until onions are sweet and a golden hue. Set aside to cool briefly.
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When onions are almost done cooking, preheat the oven to 400 degrees F and move a baking rack to the top third of the oven.
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On a lightly floured sheet of parchment, roll the chilled dough into a 12 x 15-inch rectangle.
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Place dough and parchment on a large rimmed baking sheet and set in the fridge to chill while you prep the filling. Reserve any extra dough in the fridge.
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To assemble, spread a layer of Dijon over the dough, leaving a 2-inch border. Preserving the border, scatter all bit 1/4 cup of the cheeses and the caramelized onions. (I like to leave the bay leaves in, but remove them now if you’d prefer.) Brush the border with the egg wash, fold up the long sides and brush with egg, and then fold up the short sides. Brush the top of the dough all over with egg wash, check that the corners are sealed, and finish with several twists of black pepper. Strip the leaves from the thyme sprigs and scatter thyme and remaining cheese over the onions and crust.
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Bake the tart 20 - 25 minutes or until filling is a deep golden brown and the crust is puffed up and golden. Cool 30 minutes, then slice and serve with additional thyme sprigs as a garnish.
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This tart is best enjoyed the day it’s made but leftovers will keep well for a day or two sealed at room temp.