Preheat oven to 375ยบ and line a baking sheet with parchment. Combine bread and milk in a large mixing bowl and stir until combined. Set aside. Meanwhile, in a large skillet over medium-high heat, melt 2 tablespoons butter. Add onions and garlic and cook until onions are softened, 3 to 4 minutes. Remove from heat and add to bread and milk mixture. Let cool, 5 minutes.
Add beef, egg, salt, pepper, allspice, and nutmeg to the mixing bowl and thoroughly mix together. Form 2 teaspoon-sized meatballs and place onto prepared baking sheet, 1/2″ apart, then bake until just cooked through, 10 to 12 minutes.
Sauce: In a large saucepan over medium heat, melt remaining tablespoon butter and whisk in flour. Continue to whisk 1 minute, then slowly whisk in beef broth, making sure to whisk any lumps. Season with salt and pepper. Simmer until gravy is thick enough to coat the back of a wooden spoon. Remove from heat and stir in cream, if using. Add meatballs to the gravy and toss together.
Garnish with Chives or Parsley and serve over mashed potatoes or plain cooked rice / noodles / pasta