Add the marinated chicken to the skillet. Stir-fry for 5-7 minutes until the chicken is cooked through.
Add the sliced red bell pepper and mushrooms to the skillet. Stir-fry for an additional 3-4 minutes until the vegetables are cooked through.
Pour the remaining tablespoon of soy sauce over the stir-fry. Stir well.
Dissolve cornstarch in a tablespoon of cold water. Add the cornstarch mixture to the skillet, stirring continuously for about a minute until the sauce thickens and coats the ingredients evenly.
Reduce the heat to low. Simmer the stir-fry in the sauce for an additional 2-3 minutes, allowing the flavors to meld together.
Remove from heat. Serve alone or with cooked rice or noodles for a complete meal.
Notes
The mushrooms will release liquid upon cooking - this is OK. The sauce will thicken up with the addition of corn starch.