French Onion Bread

Ingredients

• 4 1/4 cups (531g) all-purpose flour, divided
• 1/4 cup (50g) granulated sugar
• 1 (0.25oz) package (7g) or 2 1/4 teaspoons Red Star Active Dry Yeast
• 1 teaspoon (3g) kosher salt (see note)
• 1/2 cup (120g) whole milk
• 1/2 cup (120g) water
• 1/3 cup (76g) unsalted butter
• 1/4 cup (60g) sour cream
• 1 large egg (50g)
• 1/4 cup (57g) unsalted butter
• 2 pounds (908g) Vidalia onions, thinly sliced (about 8 cups)
• 1 teaspoon (3g) kosher salt
• 1/2 teaspoon (2g) granulated sugar
• 1/2 teaspoon (1g) ground black pepper
• 1/2 cup (120g) dry sherry
• 1/2 cup (113g) unsalted butter, softened
• 2 tablespoons (4g) fresh thyme
• 8 ounces (227g) Gruyère cheese, shredded and divided