Spinach Florentine Breakfast Casserole

Name: Spinach Florentine Breakfast Casserole
Ingredients: 1 ( 10 - ounce ) package frozen chopped spinach , thawed and drained; 1 tablespoon olive oil , plus more for greasing; 1 medium yellow onion , chopped (about 1 cup ); 3 clove garlic , finely chopped; 1 ( 8 - ounce ) container mascarpone cheese (about 1 cup); 4 ounces Parmesan cheese , grated on a Microplane (about 1 cup), divided; 2 teaspoons kosher salt , divided; 1 teaspoon freshly ground black pepper , divided; 10 large eggs; 1 2/3 cups half-and-half; 1/4 teaspoon crushed red pepper; 6 English muffins , split