In a medium bowl, whisk together 1.25 cups all-purpose flour, 1 tsp granulated sugar and 1/4 tsp fine sea salt. Add 6 tbs cold cubed unsalted butter. Using a pastry cutter or your fingertips, cut the butter into the flour until mixture resembles coarse crumbs with some pea-sized pieces remaining.

Bring dough together

Drizzle 3 tbs ice water over the flour-butter mixture, one tablespoon at a time, and gently mix with a fork until the dough just comes together. Add an extra teaspoon if dry. Do not overwork. Turn dough out onto a sheet of plastic wrap, press into a flat disk about 6 inches diameter, wrap and refrigerate for at least 30 minutes (up to 2 hours).

Roll dough thin

Preheat oven to 425°F (220°C). Line a baking sheet with parchment. On a lightly floured surface, roll chilled dough into a very thin rectangle or oval, about 12 x 9 inches and roughly 1/16 inch thick. Transfer to prepared baking sheet. Chill on sheet for 10 minutes if dough warmed while rolling.

Apple Filling

Prepare apples

Peel, core and very thinly slice 3 apples (about 1/8-inch thick). As you slice, place slices in a bowl and toss with 1 tbs lemon juice to prevent browning.

Sweeten and season

In a separate small bowl, mix 2 tbs granulated sugar and 1/2 tsp ground cinnamon. Toss the lemon-coated apple slices with the sugar-cinnamon mixture until evenly coated.

Arrange apples

Starting at one short end of the rolled pastry, arrange apple slices in slightly overlapping rows, working toward the other end so apples create a fan effect and cover most of the pastry but leave a 1/2-inch border around the edges. If needed, trim any excess dough so the tart is neat.

Brush with butter

Lightly brush the arranged apples with 1 tbs melted butter. This helps them brown and become glossy during baking.

Finish & Bake

Egg wash and dock

Beat 1 egg with 1 tsp cold water to make an egg wash. Brush the 1/2-inch pastry border with the egg wash (avoid brushing apples). Using a fork, dock (prick) the exposed crust lightly to keep it from puffing excessively.

Bake

Bake tart on middle rack of preheated 425°F (220°C) oven for 20–25 minutes, until pastry is golden and apples are tender and slightly caramelized at the edges. Begin checking at 18 minutes to avoid over-browning.

Glaze

While tart bakes or immediately after removing from oven, gently heat 2 tbs apricot jam with 1 tsp water in a small saucepan or microwave for 10–15 seconds until pourable. Strain if necessary. Brush warm glaze over the hot apples to give a shiny finish.

Cool slightly

Allow tart to cool on the baking sheet for 8–10 minutes so the crust firms slightly and the glaze sets.

Serve

Dust the tart lightly with 1 tbs powdered sugar if using. Slice into 6 portions. Serve each warm slice with a scoop of vanilla ice cream.