In a large bowl, combine the ground beef, breadcrumbs, finely chopped onion, minced garlic, egg, salt, and pepper. Mix gently until just combined, shape into small meatballs, and set aside.

Heat olive oil in a large skillet over medium heat. Add meatballs in a single layer and cook for about 5-6 minutes, turning occasionally, until browned. Remove and set aside.

In the same skillet, melt butter over medium heat. Add mushrooms and onions, sauté until onions are translucent and mushrooms are golden, about 5 minutes. Sprinkle in flour and stir for 1 minute.

Gradually whisk in beef broth, stirring constantly until mixture thickens (about 3-4 minutes). Remove from heat and fold in sour cream.

Return meatballs to skillet, cover, and let simmer on low heat for about 10 minutes.

Garnish with parsley and serve hot over egg noodles, rice, or mashed potatoes.