Peel and chop the potatoes into bite-sized pieces. Soak in water briefly, then drain. Microwave in a heatproof container with a loose cover for 5–6 minutes at 600W until soft. Mash with a fork while hot and mix in milk, salt, and pepper.

Trim the root of the shimeji mushrooms and chop finely. Mince the onion and garlic.

Heat butter and garlic in a pan over low heat until fragrant. Add mushrooms and onions, cooking over medium heat until softened. Mix into the potato mixture and divide into 12 portions, shaping into oval patties.

Heat vegetable oil and breadcrumbs in a pan over medium heat, stirring until golden brown. Let cool.