In a medium bowl, soak the breadcrumbs in milk for a few minutes until softened. Add the egg, grated onion, garlic, Worcestershire sauce, Dijon, salt, pepper, and thyme. Mix gently to combine.

Form the mixture into 12–14 evenly sized meatballs (about golf-ball size). Wetting your hands slightly helps them stay neat and keeps the mix from sticking.

Heat 1 tablespoon of oil in a large skillet over medium-high heat. Brown the meatballs in batches, about 2 minutes per side, until nicely seared. Transfer to a plate.

In the same skillet, reduce heat to medium and add the butter. Add the sliced onions and mushrooms. Sauté until softened and the mushrooms release their juices, about 6–8 minutes.

Sprinkle the flour over the mushroom-onion mixture and cook for 1 minute, stirring. Slowly whisk in the beef broth, ensuring there are no lumps. Add Worcestershire sauce and bring to a gentle simmer.

Nestle the browned meatballs back into the skillet, spoon some gravy over them, and simmer gently for 10–12 minutes until the meatballs are cooked through (internal temp ~160°F) and the flavors have melded. If desired, stir in the cream at the end.

Sprinkle with chopped parsley and serve the meatballs over mashed potatoes, buttered egg noodles, or a simple rice pilaf.