Thinly slice the green peppers and shiitake mushrooms.
Heat the pan over medium heat. Melt the butter and cook green peppers for about 2 minutes. Add a pinch of salt to taste.
Add the shiitake mushrooms in the pan. Cook for additional 1-2 minutes until vegetables are tender.
Turn off the heat and insert soy sauce. Mix well. Season with ground pepper and enjoy! (Add butter if you want to make it a bit more creamier).