Preparation Preheat oven to 425°F. Lightly coat a 13x9" baking pan or dish with nonstick spray. Whisk flour, baking powder, and salt in a large bowl to combine. Grate butter into dry ingredients on the large holes of a box grater, then lightly toss to coat. Freeze 10 minutes. Scatter about three-quarters of Gruyère and Parmesan over flour mixture, then add crushed red pepper flakes. Squeeze as much liquid out of spinach as possible, pull into shreds, and strew over flour mixture. Whisk buttermilk and sour cream in a large measuring glass or small bowl. Pour into bowl with flour mixture and mix with a fork just until combined. Knead in bowl with your hands until a soft cohesive dough forms (do not overmix). Scrape dough into prepared pan and press into an even layer. Using a bench scraper or knife, deeply score into 12 portions (cut down almost to pan beneath). Scatter reserved Gruyère and Parmesan over. Bake biscuits until puffed and golden, 30–35 minutes. Transfer pan to a wire rack and let cool 10 minutes before serving.