Preheat oven to 400°F. In a 10-inch cast iron skillet, melt butter over medium-high heat. Add onion; cook and stir until tender, about 3-4 minutes.
In a small bowl, mix flour, broth, milk and pepper together until smooth; stir into skillet. Bring to a boil, stirring constantly and cook until thickened, approximately 1-2 minutes.
Add turkey and vegetables; heat through.
Remove from heat and arrange quartered biscuits over turkey mixture.
Bake 15-20 minutes or until biscuits are golden brown.