• 2 1/2 cups all-purpose flour
• 2 sticks unsalted butter , cut into small pieces
• 1 teaspoon kosher salt
• 1 teaspoon sugar
• 4 to 6 tablespoons ice water
• 1 egg yolk
• 1 tablespoon heavy cream
• 2 tablespoons olive oil
• 1 tablespoon unsalted butter
• 3 large Vidalia onions , thinly sliced
• 1 tablespoon thyme
• 1 to 2 teaspoons black pepper

In a food processor, combine flour, butter, salt, and sugar; pulse until it forms a coarse meal. Continue pulsing and add water, 1 tablespoon at a time, until meal moistens into dough. Form dough into disc; wrap in plastic. Refrigerate at least 30 minutes.

Meanwhile, in a large sauté pan over medium flame, heat oil and butter. Once hot, add onions; cook slowly until golden brown, about 25 minutes. Stir in thyme, salt, and pepper. Set aside.

Preheat oven to 375°F. Roll out dough on a floured piece of parchment into 14- to 15-inch circle. Place cooled onion mixture in center, fold over edges about 2 inches. In a small bowl, combine egg yolk and heavy cream; brush mixture over edges of dough. Bake until golden brown, 20 to 25 minutes. Serve warm.