Potato, Leek and Blue Cheese Galette

Wash your hands with soap and water for 20 seconds and clean the work surface. Combine the 2 cups flour and 1/2 teaspoon salt in a mixing bowl. Add the 10 tablespoons cold, cubed butter. Rub the butter into the flour with your fingertips until the mixture resembles coarse crumbs with some pea-sized pieces remaining.

Sprinkle the 6 tablespoons ice-cold water evenly over the flour mixture. Toss with a fork, then gently press the mixture together into a dough. Stop once it holds together; do not knead it smooth. Shape into a disk about 1 inch thick, wrap and refrigerate for 45 minutes.

Wash your hands and clean flour-covered surfaces and utensils before preparing the vegetables. Scrub the potatoes under running water before peeling. Trim away damaged produce. Rinse the leeks thoroughly between their layers, then thinly slice and measure 3 cups. Separate the lettuce leaves, rinse under running water and spin dry. Refrigerate the lettuce until serving.

Put the 8 cups cooking water and 1 teaspoon salt in the saucepan and bring to a boil. Meanwhile, melt the remaining 2 tablespoons butter in the skillet over medium-low heat. Add the leeks and their 1/2 teaspoon salt. Cook for 12–15 minutes, stirring frequently, until soft but not browned. Continue cooking until any visible liquid has evaporated. Stir in the thyme and 1/4 teaspoon pepper, then transfer to a clean bowl to cool.

While the leeks cook, peel the potatoes and cut them into even 1/8-inch slices. Add them to the boiling water, return to a gentle simmer and cook for 3–4 minutes. The slices should bend slightly but still offer a little resistance in the center; do not let them become soft enough to fall apart. Drain carefully in the colander, shake gently to release trapped water and let them steam-dry and cool for about 15 minutes.

Set an oven rack in the lower third of the oven and heat to 425°F. Place a large sheet of parchment on the work surface. Use the reserved 2 tablespoons flour to dust the parchment, dough and rolling pin. Roll the chilled dough into a roughly 14-inch circle, about 3/16 inch thick.

Make sure the leeks and potatoes are no longer warm. Spread the leeks over the pastry, leaving a 2-inch border uncovered. Arrange half the potatoes over the leeks in overlapping slices, then scatter with 2 ounces of the blue cheese. Add the remaining potatoes in another even layer and scatter the remaining 2 ounces cheese over the top.

Just before serving, combine the olive oil, white wine vinegar, remaining 1/4 teaspoon salt and remaining 1/4 teaspoon pepper in a clean mixing bowl. Beat together with a fork. Tear the dried lettuce into bite-sized pieces, add it to the bowl and toss gently until lightly coated.