Cut chicken into thin slices, put in a bowl, and add soy sauce, a squeeze of lemon juice (optional), and sprinkle of salt to mix. Then leave marinating while prepping other ingredients.
Dice onion, and cut bell pepper into strips. Break shimeji mushroom apart while rinsing off the dirt.
Heat olive oil with medium heat in the pan. Toss in onion and cook until fragrant, then add chicken slices. Stir fry until golden on both sides of the chicken slices.
Turn the heat down to medium-low, and add in all veggies except mushroom. Stir fry for 2-3 minutes until the peas and pepper look softened, then add in the mushroom for another 2-3 minutes. Add salt and pepper to taste, and a little more soy sauce if desired. Plate to serve!