In a medium bowl, add the ground beef, panko, egg, garlic, ketchup, beef bouillon, mustard, and Worcestershire sauce. Mix until everything is combined well. Form the mixture into four oval-shaped patties.

Heat a skillet on medium-high heat and add 1 tablespoon of butter. Place the patties into the hot skillet and sear for 2 minutes. Flip the patties and cook for another 2 minutes. They will still be raw in the center, so remove them to a plate and set aside.

Add 2 tablespoons of butter to the skillet, then add the sliced onions. Reduce the heat to medium-low and cook the onions for 20 minutes, stirring often.

Once the onions are soft and golden, add in the sliced mushrooms and cook for another 2-3 minutes. Sprinkle in the flour and toss everything together.

Pour in the beef stock, stirring well. Return the patties to the skillet and let simmer for 5-7 minutes until they are cooked through.

Remove the patties to a plate and season the gravy with salt and pepper to taste.