In a small bowl, blend peanut butter, water, soy sauce and sugar; set aside.
Add chicken, stir-fry until firm and white, about 5 minutes. Add garlic and pepper.
Add broccoli/spinach/celery, stir-fry until bright green, about 3 minutes.
Stir in peanut butter mixture. Cook, stirring constantly until sauce is smooth, about 3 minutes.
Optionally add cumin/curry powder to taste.
Optionally serve with peanuts on top.
Serves 4.