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1
Heat a large saute pan over medium-high heat for 1 to 2 minutes. Add 2 T olive oil, then the onions and thyme. Stir to coat the onions in the oil. Once the onions have started to color, lower the heat, stir in a few pinches of salt and pepper, and cook until the onions are a golden, amber color -- about 25 to 30 minutes. Take your time with this step; the flavor is worth it.
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2
Meanwhile, heat a large saute pan or dutch oven over medium heat for 1 to 2 minutes. Add 2 T olive oil, then the garlic and cook, stirring frequently, about 30 seconds. Add the greens and toss with tongs to coat uniformly with oil. When they begin to wilt, add a few tablespoons of water, cover, and cook for about 5 minutes, stirring occasionally. Uncover and continue to cook until greens are tender and water has evaporated, another 1 to 2 minutes. Season with salt (no pepper is needed because they're naturally peppery!). Remove from pan; wrap the mustard greens in a dish towel (or paper towels) and squeeze to remove excess moisture. Don't skip this step because it will prevent a soggy crust.
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3
Mix together the caramelized onions and mustard greens; set aside.
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4
Preheat the oven to 400 degrees F.
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5
Combine ricotta, egg yolk, yogurt and 1 teaspoon of olive oil in the bowl of a food processor. Puree until smooth, then season with salt and pepper; set aside.
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6
Place the slightly thawed puff-pastry sheet on a parchment-lined baking sheet. Make an egg wash by combining 1 egg yolk with enough water to loosen. Score a 1/4-inch border around the edge of the pastry, then brush the border with the egg wash.
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7
To assemble the tart: Spread the ricotta mixture evenly on the puff pastry within the border. Sprinkle the gruyere over the ricotta. Arrange the caramelized onion - mustard greens mixture on top.
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8
Bake the tart for about 25 minutes. The crust should turn golden brown and the cheese should bubble. Lift up the crust to make sure it's crisp and fully cooked before removing from the oven. (I find it helpful to use a spatula to peek under the tart -- the crust should be pretty crisp and stiff, with slight give in the middle, when it's done.)
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9
Squeeze lemon juice over the tart and serve.
