Cowboy Breakfast Casserole With Sausage and Spinach

Name: Cowboy Breakfast Casserole With Sausage and Spinach
Ingredients: 7 slices white sandwich bread; 1 pound bulk breakfast sausage; Softened unsalted butter or nonstick vegetable cooking spray (for pan); 4 scallions, white and pale green and dark green parts separated, thinly sliced; 1 (10-ounce) package frozen spinach, thawed, squeezed dry; 8 ounces cheddar cheese, shredded (about 2 cups); 8 large eggs; 1 1/2 cups milk; 1/2 cup full-fat Greek yogurt; 1 teaspoon mustard powder; 1/2 teaspoon kosher salt; 1/2 teaspoon freshly ground black pepper; 1/4 teaspoon ground nutmeg