Heat oil in a wok or skillet over medium-high heat.
Add chicken and stir-fry until lightly browned, about 4-5 minutes. Remove and set aside.
In the same pan, add mushrooms (and optional bell peppers). Stir-fry until tender, about 3 minutes.
Add garlic and ginger; cook until fragrant, about 30 seconds.
Return the chicken to the pan. Stir in soy sauce, oyster sauce, and slurry. Cook until the sauce thickens and coats the ingredients.
Adjust seasoning with salt and pepper.
Garnish with sesame seeds and green onions before serving.