Combine all chicken ingredients in a bowl and stir to coat evenly. Set aside for 15 minutes at room temperature or up to overnight refrigerated.
While the chicken marinates, stir together all the sauce ingredients and set aside.
Heat a large nonstick pan over medium-high heat. When hot, add half the oil and the bell peppers and onion. Cook on high heat until light charred, stirring occasionally, 5-10 minutes. Transfer to a plate and set aside.
Add the remaining half of the oil and the chicken, cooking in a single layer and stirring occasionally until cooked through, about 5-10 minutes total. Add the sauce and cook until reduced and thickened. Stir in the vegetables and chili peppers, if using. Off heat, sprinkle with peanuts and green onions.