8 tablespoons (1 stick) cold, unsalted butter cut into small pieces
1 large egg yolk, lightly beaten
5–6 tablespoons ice water
For the Filling:
2 cups (1 pound) mascarpone cheese
3 tablespoons granulated sugar
Zest of 1 blood orange
2 teaspoons vanilla extract
1/4 teaspoon kosher salt
5 blood oranges
rosemary, for garnish (optional)
1 (10-inch) tart pan
For the crust:
Combine flour, sugar, and salt in a medium bowl. Add butter pieces and toss to coat in flour. Using a pastry cutter, blend the blend the butter into the flour until it is in tiny pebble-like pieces.
Add egg and 5 tablespoons of ice water and work the liquid into the flour mixture with your hands, squeezing it into the flour until it is uniformly moist and the dough holds together when you squeeze some in your hand. Add more ice water, a tablespoon at a time if needed.
Gather dough into a ball and wrap in plastic wrap. Press dough into a disk and refrigerate for at least and hour and up to 2 days.
On a lightly-floured surface roll dough into a 1/4-inch thick circle. Gently press dough into a 10-inch tart pan. Trim dough flush with the top edge of pan. Refrigerate 30 minutes.
Heat oven to 375°F. Prick bottom of dough all over with a fork. Place on a baking sheet and bake 20 minutes.
Halfway through baking, remove from oven and poke any air holes that have formed, pressing gently on the crust with the back of a fork until it is flat. Do it again when the crust comes out of the oven. Let cool completely on a wire rack. Crust can be made up to a day in advance. Cover cooled crust tightly with plastic wrap and leave at room temperature.
For the Filling:
Combine mascarpone, sugar, blood orange zest, vanilla, and salt in the bowl of a stand mixer and whisk until stiff and fluffy.
Remove all peel and white pith of the blood oranges with a pairing knife then cut into 1/4-inch thick slices.
Spread mascarpone evenly over cooled tart shell then arrange blood orange slices on top. Garnish with rosemary if you’d like. Serve immediately.