Over medium-high heat, in the pan you used to cook the chicken, sauté onion, minced garlic, pepper flake and ginger until fragrant, about 30 seconds.
Stir in the five spice, fish sauce, peanut butter and brown sugar. Stir for just a few seconds.
Add the stock and vinegar, stirring and scraping the bottom of the pan to dissolve the fond.
Reduce liquid by about half.
Taste, and add a soy sauce and white pepper. Reducing the sauce intensifies the saltiness, so don't add any until you finish the reduction.
Remove from the heat and whisk in the spring onions, sesame oil and crushed peanuts.