Trim the root of the shimeji mushrooms and separate them by hand. Thinly slice the onion.

Cut the stems off the shiso leaves and slice them into thin strips. Reserve a small amount for garnish and mix the rest with soy sauce and mayonnaise to create shiso mayo.

Season both sides of the salmon fillets with salt and pepper.

Heat salad oil in a pan over medium heat. Place the salmon skin-side down and add shimeji mushrooms and onion to the empty spaces. Stir the vegetables until coated with oil and cook the salmon for about 2 minutes until browned. Flip the salmon.

Spread shiso mayo over the salmon and top with pizza cheese. Cover the pan and cook on low heat for 1–2 minutes until the cheese melts. Season the mushrooms and onion with salt and pepper and mix.