Heat oven to 375°F. Bake biscuits as directed on bag.
In 10-inch nonstick skillet, melt butter over medium heat. Add apple slices; cook 4 to 7 minutes, stirring occasionally, until crisp-tender. Remove; cover and keep warm.
Increase heat to medium-high. In same skillet, crumble sausage; cook 4 to 5 minutes, stirring frequently, until no longer pink. Add 1 tablespoon of the flour, stirring constantly, until brown. Stir in remaining tablespoon flour. Stir in milk with wire whisk. Cook about 3 minutes, stirring constantly, until mixture thickens. Stir in onion salt, black pepper and cayenne pepper.
Split warm biscuits; place on 2 serving plates. Top with apple slices and sausage gravy.