Prepare the Meatball Mixture:

Place the panko breadcrumbs in a large bowl.

Using a box grater, grate the onion over the breadcrumbs to allow the juices to soak in.

Add the ground beef, minced garlic, egg, ketchup, crumbled bouillon cube, Worcestershire sauce, and Dijon mustard.

Mix thoroughly with your hands until well combined.

Form the Meatballs:

Roll the mixture into 20 to 22 balls, each about 1 1/2 tablespoons in size.

Cook the Meatballs:

Heat the olive oil in a large skillet over high heat.

Add the meatballs and cook until browned on the outside but still raw inside.

Transfer the meatballs from the skillet to a plate and set them aside.

Sauté the Mushrooms:

In the same skillet, melt the butter over high heat.

Add the sliced mushrooms and cook until they start to soften, about 2 minutes.

Add the salt, pepper, and minced garlic.

Cook for an additional 2 minutes, or until the garlic turns golden and the mushrooms are tender.

Prepare the Gravy:

In a bowl, mix the flour, water, Dijon mustard, and Worcestershire sauce until smooth to create the slurry.

Add the beef stock, salt, pepper, and slurry to the skillet with the mushrooms.

Stir to combine and bring the mixture to a simmer.

Simmer the Meatballs in Gravy:

When the gravy begins to simmer, return the meatballs to the skillet along with any juices that have collected.

Adjust the heat to medium and let it simmer gently for 5 minutes, or until the gravy has thickened and the meatballs are cooked through.

Serve:

Spoon the meatballs over your choice of mashed potatoes, rice, or pasta for serving.

Sprinkle with chopped parsley if desired.