Savory Bread Strata

Name: Savory Bread Strata
Ingredients: 1/2 pound (about 1/2 loaf) white, sourdough, or whole-wheat bread, preferably day-old, sliced 1/2 inch thick; 2 tablespoons olive oil; 2 red bell peppers, seeded and thinly sliced; 1 medium yellow onion, thinly sliced; 2 cloves garlic, finely chopped; 1 teaspoon chopped fresh thyme leaves; 1 1/2 teaspoons kosher salt; 1/2 teaspoon plus 1/8 teaspoon black pepper; butter for the baking dish; 1 cup (4 ounces) grated Gruyere; 4 large eggs; 2 cups whole milk