Croissant Breakfast Casserole with Sausage

Name: Croissant Breakfast Casserole with Sausage
Ingredients: 1 pound plain butter croissant; 1 pound sweet Italian sausage; 1½ tablespoons olive oil; 6 green onions ((chopped)); 1 teaspoon Italian seasoning; 8 oz sliced white or brown mushrooms; 1 red bell pepper ((diced)); 1 cup freshly chopped spinach leaves; 10 large eggs ((or about 8 jumbos)); 2 cups whole milk; 1 cup heavy cream; 4 oz freshly shredded gruyere cheese; 4 oz freshly shredded mild cheddar cheese; 1½ teaspoons kosher salt; ¼ teaspoon ground black pepper