1. **Make the Meatballs:** In a large bowl, combine the ground beef, breadcrumbs, egg, ketchup, Dijon mustard, Worcestershire sauce, onion powder, garlic powder, salt, and pepper. Mix gently with your hands just until combined (do not overwork). Form into 12–15 medium-sized meatballs.

2. **Sear the Meatballs:** Heat the olive oil in a large skillet over medium-high heat. Add the meatballs in a single layer (work in batches if needed). Fry for 4–5 minutes, turning occasionally, until browned on all sides. They don’t need to be fully cooked through yet. Remove them from the pan and set aside on a plate.

3. **Sauté Veggies:** In the same skillet (don’t wipe it out!), melt the butter. Add the sliced onions and mushrooms. Sauté for 5–7 minutes, scraping up any browned bits from the bottom of the pan, until the onions are soft and mushrooms are browned. Add the minced garlic and cook for 1 minute until fragrant.

4. **Make the Gravy:** Sprinkle the flour over the vegetables and stir well to coat. Cook for 1–2 minutes to remove the raw flour taste. Gradually whisk in the beef broth, Worcestershire sauce, and Dijon mustard. Bring the mixture to a simmer, stirring constantly until the gravy begins to thicken.

5. **Simmer & Serve:** Return the browned meatballs to the skillet, nesting them into the gravy. Reduce heat to low, cover, and simmer for 10–12 minutes, or until the meatballs are cooked through and tender. Garnish with fresh parsley and serve hot over mashed potatoes.