Cut the chicken thigh into bite-sized pieces and season both sides with a pinch of salt and pepper.

Slice the onion thinly and remove the stems from the shimeji mushrooms, breaking them apart by hand.

Heat olive oil in a frying pan over medium heat, add the chicken, and cook until browned. Then add the onion and mushrooms, stirring to combine.

Once the onion softens, add the canned tomatoes, tuna, garlic, salt, and pepper. Mix well.