In a small bowl, whisk together the minced garlic, minced ginger, gf soy sauce, sake, sugar, peanut butter and fish sauce. In a separate small bowl, whisk together the water and cornstarch.
Heat a wok over high heat. Add 1/2 the peanut oil and 1/4 teaspoon of sesame oil. Add the onions to the wok; stir fry until softened. Transfer onions to a plate. Add the chicken to the wok; stir fry until lightly browned and just cooked through. Remove the chicken from the wok and set aside on a plate. Add the peppers to the wok; stir fry until just tender. Return the chicken and onions to the pan. Add the peanut sauce to the pan along with the cornstarch mixture. Stir fry until sauce thickens. Adjust seasonings to taste with salt and freshly ground black pepper. Serve.