Overnight Croissant Breakfast Casserole

Name: Overnight Croissant Breakfast Casserole
Ingredients: 1 pound croissants (about 6 or 7); ¾ pound breakfast sausage (ground. Remove casings if bulk not available.); 1 tablespoon olive oil; 1 small onion (diced); 1/2 red bell pepper (diced); 1/2 green bell pepper (diced); 8 large eggs; 2 cups whole milk; 1 cup half and half (or heavy cream); 2 cups Gruyère (grated, divided); 1 teaspoons kosher salt; ½ teaspoon black pepper; ¼ tsp garlic powder; 1 tablespoon unsalted butter (for greasing baking dish)