Make Meatballs:
In a large bowl, combine ground beef, breadcrumbs, Parmesan, milk, egg, minced garlic, salt, and pepper. Mix until just combined. Form into 1½-inch meatballs.

Cook Meatballs:
Heat olive oil in a large skillet over medium heat. Add meatballs and cook until browned on all sides and cooked through, about 8–10 minutes. Remove meatballs and set aside.

Cook Mushrooms and Onion:
In the same skillet, add diced onion and sliced mushrooms. Sauté until softened and browned, about 5–7 minutes. Add minced garlic and cook 1 more minute.

Make Sauce:
Pour in beef broth, Worcestershire sauce, and dried thyme. Bring to a simmer and cook for 5 minutes. Stir in heavy cream and cook until sauce thickens slightly, about 3 minutes.

Combine and Serve:
Return meatballs to the skillet and coat with mushroom sauce. Simmer together for 2–3 minutes to meld flavors. Garnish with fresh parsley.