In a 10-inch cast-iron skillet, melt butter over medium-high heat. Add vegetables, onion, garlic, and sage; cook, stirring occasionally, until onion is softened, about 5 minutes. Stir flour into vegetables; gradually stir in broth, salt, and pepper until well combined. Stir in cream; cook, stirring constantly, until thickened, about 8 minutes. Stir in turkey.