Remove the stems from the shiitake mushrooms and mince the stems. Dust the inner side of the caps with flour. Mince the onion and microwave it in a heatproof container for 1 minute at 600W, then drain the moisture.

In a bowl, soak the breadcrumbs in milk. Add the minced onion, minced shiitake stems, and the remaining meat mixture ingredients. Mix until sticky and divide into 6 portions.

Place one portion of the meat mixture onto the inner side of each shiitake cap, shaping it neatly. Repeat for all 6 caps.

Heat vegetable oil in a frying pan over medium heat. Place the stuffed mushrooms meat-side down and cook until browned. Flip them over, cover with a lid, and steam on low heat for 3-4 minutes.

Top each mushroom with pizza cheese, cover, and steam until the cheese melts.