In a 4-quart Dutch oven cook red onion in 1 tablespoon hot butter over medium-high heat until tender and lightly browned. Remove from Dutch oven; set aside.
Add remaining 4 tablespoons butter to Dutch oven; stir until melted. Stir in flour, nutmeg, and allspice until combined. Add beef broth. Cook and stir until thickened and bubbly. Add meatballs and pasta; heat through. Season to taste with salt and pepper. Stir in sour cream before serving. Top servings with the sauteed red onion and dill weed.