Place the panko in a mixing bowl and grate the onion directly over it using the coarse side of a box grater. Stir to distribute the moisture evenly through the breadcrumbs.

Incorporate the ground beef, egg, crushed bouillon, garlic, ketchup, mustard, and Worcestershire sauce into the bowl. Using your hands, thoroughly blend the mixture until it becomes cohesive.

Form the meat mixture into approximately 20 to 22 meatballs, each about 1 1/2 tablespoons in size. An ice cream scoop with a release lever can help achieve uniform portions.

In a small bowl, combine the flour, water, Dijon mustard, and Worcestershire sauce, stirring until the slurry is completely smooth with no lumps.

Warm the olive oil in a large skillet over high heat. Brown the meatballs in batches, turning to sear all sides. Do not cook through; transfer them to a plate once the exterior is browned.

In the same skillet, melt the butter over high heat. Add the sliced mushrooms and sauté for about 2 minutes until they begin to soften. Add the garlic, salt, and pepper, then continue cooking for another 2 minutes until the mushrooms are tender and the garlic is lightly golden.

Pour in the beef stock along with the salt, pepper, and prepared slurry. Stir well to incorporate all the ingredients.

Bring the mixture to a gentle simmer, then return the partially cooked meatballs along with any accumulated juices to the pan. Lower the heat to medium and cook for roughly 5 minutes, or until the meatballs are fully cooked and the gravy has thickened to your liking.

Serve hot, ladled generously over a bed of mashed potatoes, rice, or noodles, and garnish with chopped parsley if using.