Prepare shimeji mushrooms by trimming the roots and separating them. Slice shiitake caps and stems thinly. Cut lemon into thin slices and quarter them. Juice the remaining lemon.

Cut salmon fillets into four pieces. Season with salt and pepper, then coat with cornstarch.

Mix marinade ingredients (sugar, soy sauce, water, olive oil, and lemon juice) in a bowl.

Heat 1 tbsp olive oil in a pan over medium heat. Cook shimeji and shiitake mushrooms until softened, then add them to the marinade.

Heat the remaining 1 tbsp olive oil in the pan. Cook salmon pieces until golden brown on both sides and cooked through.