Preheat oven to 400°. Melt butter in a 10-in. cast-iron or other ovenproof skillet over medium-high heat. Add onion; cook and stir until tender, 2-3 minutes. , In a small bowl, mix flour, broth, milk and pepper until smooth; stir into pan. Bring to a boil, stirring constantly; cook and stir until thickened, 1-2 minutes. Add the turkey and frozen vegetables; heat through. Arrange biscuits over stew. Bake until biscuits are golden brown, 15-20 minutes.