In a bowl, mix the grated onions, panko breadcrumbs, garlic powder, onion powder, Worcestershire sauce, soy sauce, mustard powder, ketchup, and a pinch of salt. Let the mixture rest for 5 minutes to absorb moisture. Add the ground beef and egg, stirring just until blended. Form the mixture into 18-22 meatballs, each about 1 ½ tablespoons in size.

Heat a large skillet over medium-high heat and add 1 tablespoon of olive oil. Cook the meatballs in a single layer, flipping halfway, until browned on all sides, about 6-9 minutes. It’s okay if they aren’t fully cooked through yet. Remove the meatballs to a plate, and repeat with remaining meatballs, adding more oil if needed.

Add 1 tablespoon of butter to the skillet. Once melted, toss in the chopped mushrooms and cook until they’re nicely browned, about 3-4 minutes. Add the sliced onions and cook for another 3 minutes, followed by the garlic for 30 seconds until fragrant.

Melt the rest of the butter in the skillet, then stir in the flour and cook for about 1 minute, stirring constantly. Slowly add the beef broth, whisking continuously until smooth. Mix in the Dijon mustard, soy sauce, and Worcestershire sauce. Let the sauce simmer for 3 minutes, allowing it to thicken slightly.

Add the meatballs back into the skillet with any juices from the plate. Let everything simmer together for about 4 minutes. If the gravy becomes too thick, you can thin it out with a splash of water. Adjust seasoning to taste.

Serve the meatballs with the mushroom gravy over mashed potatoes or egg noodles.