Fragrant and sweet browned onions are the foundation of this savory custard tart. It may have rich ingredients, but it won't make you feel heavy. I love how aged or smoked Gouda adds some extra depth of flavor to the tart, but Gruyère or Comte are good substitutes. If you do not have whole wheat pastry flour you can use all whole wheat flour instead. You can make the pastry dough and cook the onions a day ahead. The pastry dough will keep in the refrigerator covered in plastic wrap for 3 days. Or, freeze it for up to 3 months. The onions are best eaten within 24 hours of making them. Onion tart is perfect for brunch, lunch, a light supper or as an appetizer for a cocktail party. It is a very versatile food you can make all year long. Special equipment: 9-inch (23 cm) tart pan with removable bottom. You can use any shape, square, circle or an 11 x 8.5-inch ( 28.5 x 20 cm) rectangle tart pan. Stand mixer (optional) Rimmed sheet pan large enough to hold your tart pan. This recipe is slightly adapted from Vegetable Literacy by Deborah Madison.
- ¾ cup plus 2 Tb (123 g) Whole wheat flour
- ¼ cup (38 g) whole wheat pastry flour
- 6 TB (106 g) cold butter, cut into small pieces
- ½ tsp Kosher salt
- 3 TB ice water
- 1 TB olive oil for frying the prosciutto
- 2 thin slices of prosciutto or 2 pieces of bacon (optional)
- 2 TB (7 g) butter
- 1½ lbs (725 g) white onions, diced
- 2 tsp fresh thyme or rosemary minced
- Kosher salt
- Freshly ground black pepper
- 3 eggs
- ½ cup (125 ml) milk
- ½ cup (125 ml) crème fraîche or heavy cream
- 1 cup (70 g) aged or smoked Gouda cheese, grated using the large holes on a box grater
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Make the dough by hand or use a stand mixer.
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By hand: Add the two types of flour and Kosher salt to a large bowl. Add the butter and mix the butter and flour with your hands. Press down on the butter between your thumbs and fingers to break up the pieces and press into the flour. Continue to do this until the butter and flour are mixed together and looks like pebbles.
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Add the water and mix together with your hands. Add more water if it looks and feels dry, about a teaspoon at a time.
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Gather the dough and turn it out onto the counter. Press together and form a flat disc in the shape of your tart pan, about an inch (2.5 cm) thick. A circle, square or rectangle shape.
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Wrap in plastic wrap and refrigerate for one hour or more. The tart dough can be made 3 days in advance and kept wrapped in the refrigerator.
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By stand mixer: Add the flour to the bowl of a stand mixer. Add the pieces of butter and mix the butter and flour on low until the flour looks like pebbles. Add the water and stir on low speed until just mixed together. Add more water if the pastry dough looks dry. Be careful not to over-mix the dough. Turn the dough onto a counter and shape into a flat disc into the shape of your tart pan. Cover with plastic wrap and refrigerate for one hour or more.
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Preheat the oven to 400°F / 200°C. If you have a baking stone place it in your oven on the middle rack. For a crispy bottom crust, you want to heat the stone in the oven for an hour before baking.
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If using the prosciutto, add 1 tablespoon of extra virgin olive oil to a skillet and turn the heat to medium-high. Slice the prosciutto into strips, about a 1/2-inch (1 cm) wide and not longer than 2-inches (5 cm) long. When the skillet is hot, add the sliced prosciutto and cook until the strips are brown and crispy. Stir occasionally to prevent the strips from sticking and burning. About 5 minutes. Turn off the heat and spoon the prosciutto slices on a plate. Set aside.
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If you are using bacon add the two pieces of bacon to a hot and dry skillet and cook the bacon until they are brown and crisp. Turn the pieces over every now and then for even browning. Remove the bacon from the pan onto a plate lined with paper towels and pat dry. When the bacon is cool, crumble them into bite-size pieces. Set aside.
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Wipe out the skillet with a paper towel. Turn the heat to medium and add the butter. When the butter is melted and stops sizzling, add the diced onions. Stir to coat the onions with butter. Add the thyme, a pinch of Kosher salt and a couple of grounds of fresh black pepper. Stir to mix.
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Cook the onions on medium to medium-low until they are very soft and lightly browned, not caramelized, but starting to go in that direction. This is a slow process, about 25 minutes. The onions will be very soft and translucent with an even light brown color. While cooking, occasionally stir the onions for even browning and prevent them from sticking and burning. Taste for seasoning and add more thyme, Kosher salt or black pepper if needed. Turn off the heat and cool.
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Remove the tart dough from the refrigerator and let it rest on a lightly floured work surface for 10 minutes. Pound the dough with your rolling pin a few times to relax and shape the dough. Roll the dough in the shape of your tart pan to about a ¼-inch (.5 cm) thickness, and large enough to fit the shape of your tart pan with a slight overhang. For a 9-inch (23 cm) round tart pan the diameter should be around 12-inches (30 cm).
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Drape the dough into your tart pan and trim the edges to an inch (2.5 cm) overhang. Fold the edge of the dough inward and press along the sides and bottom of the pastry to fit the dough into the pan. The height of the tart is equal to the height of the pan. Place the tart pan on a rimmed sheet pan and loosely cover the tart with plastic wrap. Chill the tart in the refrigerator for 15 - 20 minutes.
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In a small bowl, whisk together the eggs, milk and crème fraîche.
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When the onions are cool and just before you want to assemble the tart, add the grated cheese, onions, and prosciutto if using, to the egg mixture. Stir to mix.
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Remove the tart from the refrigerator and add the egg mixture. Even out the filling and place in the oven. Bake until the tart is golden brown and set in the middle, about 45-50 minutes.
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Remove from the oven and cool to room temperature. Remove the side of the tart pan by resting the tart on top of a large can, (like canned tomatoes), and slide the side rim down. Make sure the crust is not sticking anywhere along the rim before you slide it off.
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Serve warm or room temperature.