Cut the onion in half; slice one half thinly and finely chop the other half. Microwave the chopped onion for 1 minute at 600W, then let it cool. Trim and separate the shimeji mushrooms. Mince the garlic after removing its core.

In a bowl, soak breadcrumbs in milk. Add ground meat, egg, microwaved onion, salt, and pepper. Mix until sticky. Divide into 4 portions, shape into patties, and indent the center slightly.

Heat olive oil and garlic in a frying pan over low heat. Once fragrant, add the patties and cook on medium heat until browned on both sides, about 2 minutes per side.

Add the sauce ingredients (☆) to the pan and stir to dissolve the miso. Add shimeji mushrooms and sliced onion, mix well, cover, and simmer on low heat for 10 minutes, stirring occasionally.