Author Notes: If I had to choose a favorite green, it would be mustard greens, which often takes a backseat to the more common swiss chard, kale, and collards. I combined the best elements of several recipes from two of my favorite cookbooks, Sunday Suppers at Lucques and Zuni Cafe, to create a tart that would highlight the bright, peppery notes of this unsung ingredient.
This tart is all about building layers of flavor. It starts with a base of ricotta and greek yogurt; a layer of gruyere cheese; and then a mixture of caramelized onions and sautéed, garlicky mustard greens. This same filling could work in a tart shell or even a galette, but in my mind the flaky, buttery puff pastry makes the perfect canvas. Note: the proportions below are for an 8 oz sheet of puff pastry; I'd recommend doubling the filling if you're using a sheet that's 14 oz or larger. - EmilyC
- 1 sheet (about 8 oz) frozen all-butter puff pastry, slightly thawed
- 2 egg yolks
- 4 tablespoons plus 1 teaspoon olive oil
- 3/4 pounds yellow onion, thinly sliced (about 3 cups)
- 2 teaspoons thyme, finely chopped
- 1 bunch mustard greens, stems and center ribs discarded, leaves cut into wide ribbons
- 2 large garlic cloves, minced
- 1/4 cup whole milk ricotta
- 2 tablespoons whole milk greek yogurt
- 1/2 cup gruyere cheese, shredded
- kosher salt and pepper, to taste
- 1/2 lemon, for juicing
- Heat a large sauté 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.
- Meanwhile, heat a large sauté 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.
- Mix together the caramelized onions and mustard greens; set aside.
- Preheat the oven to 400 degrees F.
- 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.
- 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.
- 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.
- 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.)
- Squeeze lemon juice over the tart and serve.