In a large bowl, combine the sliced chicken, black pepper, cornstarch, and a pinch of salt. Mix well to evenly coat the chicken.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the chicken and stir-fry for 5-7 minutes until cooked through and no longer pink. Remove the chicken from the skillet and set aside.
In the same skillet, add the remaining tablespoon of oil. Add the red and yellow bell peppers and snap peas. Stir-fry for about 2-3 minutes until they start to soften.
Add the minced garlic and grated ginger to the vegetables, stir-frying for an additional 1-2 minutes until fragrant.
Return the cooked chicken to the skillet with the vegetables. Drizzle the soy sauce and oyster sauce (if using) over the stir fry, stirring the ingredients to combine well. Cook for another 2 minutes for the flavors to meld.
Drizzle with sesame oil, give it one final toss, and adjust salt if necessary.
Serve immediately, garnished with fresh cilantro.
Notes
Adjust the amount of black pepper to taste for desired spiciness.