Cut the chicken into one-inch pieces. Cut the onion in large pieces, and the peppers should be sliced large as well.
Preheat a saute pan or wok and put the oil in when the pan has started to smoke. It will be really hot, and that's what you want. Add the vegetables and saute them until the onion is translucent, stirring all the while. Add the chicken and saute it making all pieces are cooked through before adding the Hoisin sauce.
Add the hoisin sauce and chili flakes, stir to combine, then cover and reduce the heat to medium. Cook covered, about 5 minutes. Stir in the peanuts, then plate up.
Garnish with more peanuts, sesame seeds and chopped spring onion.