Mix the sambal paste, soy sauce, oyster sauce, and brown sugar in a bowl to make the marinade.
Marinate the chicken pieces in the sambal marinade for at least 30 minutes.
Heat the vegetable oil in a pan over medium heat. Add the minced garlic and sliced onion, and sauté until fragrant.
Add the marinated chicken to the pan and cook until the chicken is browned on all sides.
Add the sliced red bell pepper to the pan and stir-fry for a few minutes until slightly softened.
Season with salt to taste.
Garnish with chopped cilantro before serving.