Mix the meatball base. In a large bowl, whisk the egg with the panko until the crumbs absorb the liquid. Add the ground beef, Worcestershire, Dijon, ketchup, onion powder, garlic powder, dried onion flakes, and beef bouillon powder. Mix with your hands just until evenly combined — don't overwork it.
Roll the meatballs. Portion the mixture and roll between your palms into 1–2 inch meatballs. Keep them the same size so they cook evenly.
Sear the meatballs. Heat the olive oil in a large skillet over medium-high heat. Add the meatballs and sear per side, turning to brown about four sides. Remove and set aside.
Cook the onions and mushrooms. In the same skillet, add the sliced onion and mushrooms. Cook for until softened, scraping up any browned bits from the bottom of the pan.
Build the gravy. Sprinkle in the flour and stir to form a roux. Cook for , stirring occasionally. Pour in the beef broth and add the Better Than Bouillon paste. Whisk smooth and bring to a gentle simmer.
Simmer everything together. Return the meatballs to the skillet, lower the heat, and simmer in the gravy for until the meatballs are cooked through and the sauce thickens. Serve over mashed potatoes or rice.