Chop the onions, cut the chicken breast into bite-sized pieces, quarter the mushrooms, wash and drain the bean sprouts, and cut the red peppers into narrow strips.
Marinate the chicken pieces in 2 teaspoons of soy sauce and refrigerate while you prepare the other ingredients.
Heat 2 teaspoons of neutral oil in a large frying pan over medium-high heat. Fry the marinated chicken for 4-5 minutes, stirring occasionally, until cooked through (74 °C / 165 °F). Remove from the pan and set aside on a plate.
In the same pan, fry the mushrooms, peppers, and onions over medium heat for 5 minutes, stirring occasionally, until softened.
Mix 4 teaspoons of cornstarch with 400 ml of vegetable stock until smooth. Pour this mixture into the pan with the vegetables and bring to a boil over medium-high heat, stirring constantly until the sauce thickens, about 1-2 minutes.
Add the drained bean sprouts and cook for 3 minutes, stirring gently. Season the mixture with 12 teaspoons of soy sauce, 1 teaspoon of sambal oelek, and 1 pinch of sugar. Stir to combine.
Return the cooked chicken pieces to the pan, add 2 teaspoons of sesame oil, and stir gently until combined. Remove from heat.
Serve with 6 tablespoons of cooked rice per person.