Cheesy Biscuits and Gravy Casserole

Name: Cheesy Biscuits and Gravy Casserole
Ingredients: 5 tablespoons unsalted butter, divided; 6 ounces pepper jack cheese; 1 pound uncooked pork breakfast sausage, casings removed; 1/4 cup all-purpose flour; 3 cups whole milk; 1 1/2 teaspoons kosher salt, plus more as needed; 3/4 teaspoon freshly ground black pepper; 1/4 teaspoon red pepper flakes; 1/4 teaspoon ground nutmeg (optional); 1 (16-ounce) tube flaky biscuit dough (8 large biscuits); Flaky salt (optional); 1 small bunch fresh chives (optional)