Heat a large skillet over medium-high heat. Cook and stir sausage in the hot skillet until browned and crumbly, 5 to 7 minutes. Set cooked sausage aside. Melt butter in a Dutch oven over medium-high heat. Stir in turkey, drippings, garlic, thyme, parsley, mustard powder, salt, and pepper. Add water and bring to a boil, about 5 minutes. Stir in browned sausage, potatoes, mushrooms, and onion. Reduce temperature to medium-low and simmer until potatoes are tender, stirring soup every 10 minutes, for 30 to 45 minutes. Stir kale into the soup and reduce temperature to low. Simmer until kale wilts, about 5 minutes. Stir sour cream and milk into the soup, cover, and simmer 10 minutes more.