There is little simpler or more wonderful than this tart, whether you serve it as a starter, for lunch. While mustard may not be what you think of as a mix-in with tomatoes, it’s what makes this tart remarkable. The crust holds sturdy greens, ripe tomatoes, dabs of sweet ricotta, and egg custard, all compatible together and livelier with the mustard. I like grainy French mustard-sometimes called old-fashioned, or moutarde à l’ancienne - but you can go with smooth Dijon if you prefer (or if that’s what’s in the fridge). What’s really important is its freshness: The mustard should have good color (it shouldn’t have gone brown) and bright flavor.

A word on the greens and tomatoes
If you’d like, you can use baby kale or spinach instead of large-leaf greens. And although this tart is best when summer’s tomatoes are the ripest, I often turn to the recipe when the season is past. The oven’s heat sweetens the tomatoes and makes them more satisfying than you’d expect.

Ingredients:

Filling

One 9 to 9½-inch tart shell made with Pâte Brisée (recipe below), partially baked and cooled

2 teaspoons olive oil, plus more for brushing
2 cups (about 100 grams) packed finely shredded greens, such as chard, kale, spinach, or arugula
Fine sea salt and freshly ground pepper
About 1 pound (454 grams) tomatoes (3 to 4 medium or a mix of medium and cherry or grape tomatoes)
½ cup (125 grams) ricotta (whole-milk or low-fat)
1 large egg yolk
3 large eggs
6 tablespoons (90 ml) heavy cream
3 tablespoons grainy mustard (preferably French; see headnote)

Pate Brisee

1 1/4 cups (170 grams) all-purpose flour
1 teaspoon sugar
1/2 teaspoon fine sea salt
6 tablespoons (3 ounces; 85 grams) very cold unsalted butter, cut into bits
1 large egg
1 teaspoon ice water

Instructions:

Center a rack in the oven and preheat it to 425 degrees F.
Place the partially baked tart shell on a baking sheet lined with parchment paper or a silicone baking mat.

To make the Filling:

Warm the olive oil in a large skillet, preferably non-stick, over medium heat. Toss in the greens, season with salt and pepper, and cook, stirring, until they wilt and soften, 1 minute or less. (The wilted greens can be kept at room temperature for up to 2 hours.) Scrape them into the crust, leaving any oil in the skillet.

Remove the little core at the top of each medium tomato and slice the tomatoes. If you’ve got cherry or grape tomatoes, halve them. Lay the tomatoes out on a double layer of paper towels, cover with more paper towels and pat away the excess moisture.

Mix the ricotta with the egg yolk and season with salt and pepper.

Whisk together the eggs, cream, and mustard; season with salt and pepper. Pour into the crust. Add the tomatoes, arranging them so that they’re fairly evenly distributed. Finish by spooning dollops of the ricotta mixture over the top.

Bake for about 30 minutes, until the filling is puffed and firm in the center; a skewer inserted into the center should come out clean. Transfer the baking sheet to a rack and let the tart cool for at least 15 minutes before serving, or allow it to come to room temperature—it’s good at any temperature.

Just before serving, if you’d like to give the tart a little shine—the tomatoes look good with a gloss — brush a little olive oil over the surface.

Storing:

The tart is best served the day it is made, but if you have leftovers, cover and keep them in the fridge — they’ll make a good snack the next day.

Working ahead:

You can prebake the crust up to 2 months ahead and keep it, wrapped airtight, in the freezer. You can prep the ricotta and mustard mixtures ahead and refrigerate for up to 6 hours. You can wilt the greens a couple of hours ahead.

To make the Pate Brisee:

Put the flour, sugar, and salt in a food processor and pulse to blend. Scatter the butter over the dry ingredients and pulse until the butter is cut in coarsely - you'll have some pieces the size of oatmeal flakes, some the size of peas. Beat the egg and water together and add it in three additions pulsing after each bit goes in. Then whirl the food processor until the dough forms moist clumps and curds - you're aiming for a moist dough that holds together when pinched.

Shape the dough into a disk, pat it down to flatten, and put it between sheets of parchment paper. Roll the dough out evenly, turning it over frequently, and lifting the paper often so that it doesn't roll into the dough and form creases. Roll the dough into a circle that's about 11 inches in diameter. If the dough is cool, use it right away. If not cool, slide it, still in the paper, onto a baking sheet and refrigerate or freeze for up to 2 hours.

Butter a 9- to 9 1/2-inch tart pan with a removable bottom. Let the chilled dough rest on the counter until it's just pliable enough to bend without breaking. Remove both sheets of paper, fit the dough into the tart pan and trim the excess dough even with the edge of the pan. (If you'd like, you can fold the excess over and make a thicker wall around the sides of the tart.) Prick the crust all over with the tines of a fork and freeze for at least 30 minutes - an hour or two is better - or up to 2 months (well wrapped) before baking.

To partially bake the Crust:

Center a rack in the oven and preheat it to 400 degrees F. Butter the shiny side of a piece of foil and fit it snugly against the crust. Fill it with dried beans or rice.

Bake the crust for 25 minutes, then carefully remove the foil and weights. If the crust has puffed, press it down gently with the back of a spoon. Transfer the crust to a rack to cool.