Spinach and Mushroom Strata

Name: Spinach and Mushroom Strata
Ingredients: 12 ounces crimini or button mushrooms, thinly sliced; 1 teaspoon butter; 8 ounces fresh spinach or 1 box frozen, chopped spinach thawed; 1/3 cup chopped sun-dried tomatoes; 1/2 cup grated gruyere cheese + extra for sprinkling; 1/4 cup parmesan cheese + extra for sprinkling; 5 large eggs; 1 1/2 cups milk; 1 teaspoon mustard powder; 1/8 teaspoon cayenne pepper or more to taste; 3/4 teaspoon garlic salt; 1/2 teaspoon black pepper; 3/4 teaspoon kosher salt, divided; 4 cups stale italian or sourdough bread, crusts removed, cut into 1/2 cubes