Place the chicken thighs, skin side down, in a hot frying pan. Cook over high heat until the skins render fat and turn brown. Flip to brown the other side. Pour off the fat, if you prefer.
Add the onion, ginger, garlic and sambal oelek (or chilis). Pour in the soy sauce, fish sauce and rice wine vinegar.
Stir the chicken with the sauces and spices. Bring to the boil then lower the heat. Cover and cook over low heat for 30 to 40 minutes, depending on the size of the chicken thighs.
Add the peanut butter. Stir to dissolve the peanut butter in the hot liquid.
Boil, uncovered, until the sauce is rich and thick. Add black pepper. Season with more fish sauce, if needed.
Sprinkle with finely scallions.
Serve the chicken with spicy peanut sauce with rice or unleavened bread.