Potato, Sausage, and Spinach Breakfast Casserole

INGREDIENTS

• 16 large eggs
• 1 1/4 cups heavy cream
• 1 1/2 teaspoons kosher salt plus more
• 3/4 teaspoon freshly ground black pepper plus more
• 5 tablespoons unsalted butter, divided
• 12 ounces fresh breakfast sausage links
• 2 cups sliced leeks, white and pale-green parts only
• 2 russet potatoes (1 1/2 pounds), peeled, shredded, lightly squeezed
• 1 pound frozen spinach, thawed, squeezed dry, roughly chopped
• 1 1/2 cups (4 ounces) grated Gruyère
• 1/2 cup (1 1/2 ounces) grated Parmesan