Combine the bread and 1⁄4 cup of the milk in a bowl and soak for 5 minutes. Drain and use your hands to squeeze out some of the liquid absorbed by the bread (you want it to be moist, not sopping). In a mixing bowl, combine the bread with the beef, turkey, onion, garlic, nutmeg, salt, and pepper. Gently mix, then form into golf ball-sized meatballs. Heat the butter in a large nonstick sauté pan or skillet over medium heat. Add the meatballs and cook, turning occasionally, for about 10 minutes, until nicely browned on all sides and cooked through. Remove the meatballs to a plate and reserve. Add the flour to the pan, stirring with a whisk or a wooden spoon to incorporate evenly into the remaining fat. Add the stock slowly, stirring frequently to keep lumps from forming. Add the remaining 1⁄4 cup milk and simmer for about 3 minutes, until the flour begins to thicken the liquid. Stir in the yogurt, then return the meatballs to the pan, cooking for about 10 minutes longer, until the sauce clings tightly to the meatballs. Serve by themselves, or with boiled potatoes or lightly buttered noodles. Pass the marmalade at the table, if using.