• 1 1/4 cup all-purpose flour
• 1 stick unsalted butter , very cold and cut into pieces
• 1/2 tsp. salt
• 1/2 tsp. sugar
• 3 to 4 Tbsp. nonfat milk or very cold water

• 3 Tbsp. olive oil
• 4 Spanish onions , cut into 1/2-inch strips
• 4 strips bacon , cut into 1/2-inch pieces
• 2 tsp. chopped fresh rosemary
• 1 tsp. kosher salt
• 1/2 tsp. freshly ground pepper
• 8 ounces Camembert cheese , cut into 1/4-inch strips

Combine flour, butter, salt, and sugar in a food processor; pulse 6 to 8 times until mixture resembles coarse meal. Add milk 1 tablespoon at a time and pulse just until mixture forms a ball. Remove dough and shape into a 5-inch disk; wrap in plastic wrap and refrigerate at least 1 hour.

Meanwhile, prepare filling: In 2 large sauté pans (or in batches), divide oil and onions, and cook slowly over medium-low heat until golden brown, 20 to 30 minutes. Add bacon and cook until lightly browned, about 5 minutes. Stir in rosemary, salt, and pepper. Remove from heat and cool in pan.

Preheat oven to 375°. Unwrap dough and let sit on floured parchment paper until just pliable, 10 to 15 minutes. Roll dough with a floured rolling pin only in forward and backward motions; rotate dough a quarter turn with every roll, making sure dough is not sticking, until it is 1/4- to 1/8-inch thick and 11 inches in diameter.

Place half of Camembert cheese in center of dough in a pattern resembling spokes of a wheel. Top with cooled onion mixture, leaving a 1 1/2-inch border. Place remaining cheese strips on onions in the same pattern, and carefully fold up edges of crust, pleating slightly. Slide tart with parchment onto a large, rimless baking sheet; place on the center rack of the oven. Bake until crust is golden, 25 to 30 minutes. If onions are browning too quickly, cover them with foil (don't cover crust). Remove tart from oven; cool at least 20 minutes before moving off baking sheet or serving.