In a large bowl, combine ground meat, onion, egg, breadcrumbs, and a pinch of salt and pepper. Mix thoroughly until well combined.

Shape the mixture into 4 equal-sized patties.

Heat a pan over medium heat and cook the patties until browned on both sides and cooked through, about 4-5 minutes per side.

In the same pan, add shimeji and maitake mushrooms. Sauté until softened.

Add ketchup and tonkatsu sauce to the mushrooms. Stir well and simmer for 2-3 minutes.