Mix. In a large bowl, combine the beef, panko, grated onion, 1 clove garlic, egg, ketchup, crumbled bouillon, 1/2 tsp Worcestershire, and 3 tsp Dijon. Mix gently until just combined.
Shape. Roll into about 16 meatballs the size of golf balls.
Brown. Heat the oil in a large deep skillet over medium heat. Brown the meatballs for 5 to 6 minutes, turning, until browned all over. Transfer to a plate. They will not be cooked through yet.
Saute the mushrooms. Melt the butter in the same skillet, add the mushrooms, and cook 5 to 6 minutes until golden and soft. Add 2 cloves garlic and cook 1 minute.
Make the gravy. Sprinkle in the flour and stir for 1 minute. Slowly pour in the broth, whisking, then add the water, 2 tsp Dijon, 2 tsp Worcestershire, salt, and pepper. Simmer 3 to 4 minutes until slightly thickened.
Finish. Return the meatballs and any juices to the skillet, spoon the gravy over, cover, and simmer on low 8 to 10 minutes until the meatballs reach 160 degrees F.
Serve. Serve over mashed potatoes, rice, or pasta and sprinkle with parsley.