Coat a large skillet with nonstick spray and warm over med-high heat. Add the onion and cook, stirring, for 5 minutes, or til softened. Add the chicken to the skillet and cook 3-4 minutes on each side or til browned.
Meanwhile in a blender combine the reserved tomato juice, salt, paprika, pepper, peanut butter, cilantro, garlic, and red-pepper flakes. Blend til smooth. Pour over the chicken and add the tomatoes.
Bring to a boil over high heat. Reduced heat to low, cover, and simmer for 25 minutes. Serve over rice or barley.