Spinach and Cheese Slab Biscuits

Name: Spinach and Cheese Slab Biscuits
Ingredients: Nonstick vegetable oil spray or unsalted butter (for pan); 4½ cups (563 g) all-purpose flour; 2 Tbsp. baking powder; 2 tsp. Diamond Crystal or 1¼ tsp. Morton kosher salt; ½ cup (1 stick) chilled unsalted butter; 8 oz. Gruyère, Comté cheese, and/or aged white cheddar, coarsely grated; 2 oz. Parmesan, finely grated; ¼ tsp. crushed red pepper flakes; 1 10-oz. package frozen chopped spinach, thawed; 1 cup buttermilk; 1 cup sour cream (not low-fat)