Heat 1 tablespoon of oil in a wok or large skillet over medium-high heat.
Add the chicken and stir-fry for 4-5 minutes until lightly browned. Remove and set aside.
In the same pan, add the remaining oil. Stir-fry mushrooms and bell pepper for 3 minutes until tender.
Add garlic and ginger; stir until fragrant, about 30 seconds.
Return the chicken to the pan. Add soy sauce, oyster sauce, and chili paste; mix well.
Stir in the cornstarch slurry to thicken the sauce, cooking for 2 minutes.
Remove from heat. Garnish with green onions and sesame seeds. Serve hot over rice or noodles.