Make the crust: Pulse flour, powdered sugar, and salt in a food processor. Add cold butter and pulse until coarse crumbs form. Add egg yolk and pulse until dough clumps together.

Form dough into a disk, wrap in plastic, and chill for at least 30 minutes.

Preheat oven to 375°F (190°C). Roll out dough and press into a 9-inch tart pan. Prick the bottom with a fork and freeze for 15 minutes.

Line crust with parchment and pie weights. Bake for 20 minutes. Remove weights and paper, bake for another 5-10 minutes until golden. Cool completely.

Make the filling: In a double boiler, whisk together grapefruit juice, lemon juice, zests, sugar, eggs, and egg yolks.

Cook over simmering water, whisking constantly, until thick enough to coat a spoon (about 170°F/77°C).

Remove from heat. Whisk in the room temperature butter one tablespoon at a time until fully emulsified and glossy.

Strain the curd through a fine-mesh sieve into the cooled tart shell. Smooth the top.

Chill in the refrigerator for at least 4 hours, or overnight, until set firm.

Just before serving, remove from pan and garnish with fresh grapefruit segments, lemon zest, and dots of berry coulis.