In a large bowl, combine the ground beef, breadcrumbs, finely chopped onion, milk, egg, garlic powder, salt, and pepper. Mix until just combined—don’t overmix, or they might get tough! Shape the mixture into meatballs, about the size of golf balls.

Heat a large skillet over medium heat with a drizzle of oil. Add the meatballs in batches, ensuring they have enough space around them. Brown them on all sides for about 5-7 minutes. Remove and set aside.

In the same skillet, toss in the sliced mushrooms. Sauté for about 5 minutes until they’re golden and tender.

Sprinkle the flour over the mushrooms and stir for a minute. Gradually pour in the beef broth while stirring to avoid lumps. Add in the Worcestershire sauce and heavy cream, and bring to a low simmer. Let this bubble gently until it thickens, about 3-5 minutes.

Gently add the browned meatballs back into the skillet with the gravy. Let them simmer together on low for about 10 minutes.