Soak panko bread crumbs in milk for 10 minutes. Sauté onions in olive oil and 1 tbsp butter until translucent.
Add garlic and oregano, and cook for 1-2 more minutes.
In a large bowl, mix the ground beef, sautéed onions, garlic, oregano, salt, pepper, egg, and soaked bread crumbs.
Form into 20 meatballs.
In the same skillet, brown the meatballs on all sides, working in batches. Transfer to a baking sheet and keep warm in the oven.
In the skillet, melt 4 tbsp butter, whisk in flour, and cook until golden brown.
Slowly stir in warmed beef broth and cook until the sauce thickens slightly.
Stir in heavy cream, Worcestershire sauce, and Dijon mustard. Simmer the sauce until creamy.
Return the meatballs to the skillet, cover, and simmer for 10 minutes. Serve with noodles or mashed potatoes.