We (Scott and Michelle) love this recipe. I found it in a reddit comment a few years back. We substitute the ground beef in the meatballs with ground chicken, and I honestly think it makes the recipe better. Instead of chopping the onion, I just grate it on the large holes of a box grater.
In a medium sized bowl combine ground beef, Panko, parsley, allspice, nutmeg, onion, garlic powder, pepper, salt and egg. Mix until combined.
Roll into 12 large meatballs or 20 small meatballs. In a large skillet, heat olive oil and 1 Tablespoon butter. Add the meatballs and cook turning continuously until brown on each side and cooked throughout. Transfer to a plate and cover with foil.
Add 4 Tablespoons butter and flour to skillet and whisk until it turns brown. Slowly stir in beef broth and heavy cream. Add Worcestershire sauce and dijon mustard and bring to a simmer until sauce starts to thicken. Salt and pepper to taste.
Add the meatballs back to the skillet and simmer for another 1-2 minutes. Serve over egg noodles or rice.