Spinach and Onion Braided Loaf

Ingredients

• 4 1/4 to 4 1/2 cups (531 to 562g) all-purpose flour, divided
• 1 tablespoon (12g) granulated sugar
• 1 (0.25oz) package (7g) or 2 1/4 teaspoons Platinum Yeast
• 2 teaspoons (6g) kosher salt (see note)
• 1/2 cup (120g) water
• 1/2 cup (113g) unsalted butter
• 1/2 cup (120g) whole milk
• 1 large (50g) egg (room temperature)
• 1 large (19g) egg yolk (room temperature)
• 1 large (50g) egg
• 1 tablespoon (15g) water
• 1 teaspoon (5g) olive oil
• 1/2 cup (65g) chopped yellow onion
• 1 clove (5g) garlic, minced
• 9 ounces (255g) fresh spinach, chopped (about 8 cups)
• 1 teaspoon (3g) kosher salt (see note)
• 1/2 teaspoon (1g) ground black pepper
• 1/4 teaspoon dry mustard
• 1/8 teaspoon paprika
• 1/8 teaspoon ground red pepper
• 1/2 cup (57g) freshly grated Gruyère cheese
• 1/2 cup (57g) freshly grated sharp white Cheddar cheese