In a blender or food processor, blend together the nectarine (or peach), cilantro, sugar, chili powder, cumin powder, salt, lime juice, garlic, pepper and red pepper flakes together until smooth. Reserve 3/4 cup of the nectarine puree and pour over the boneless chicken. Move around the chicken so that each piece of chicken is covered in the marinade. Place the chicken in the fridge until ready to grill. Pour the rest of the nectarine puree into a medium sized bowl (for the salsa).
Salsa
Cut and chop the nectarine, cilantro, onion and peppers and place in the salsa bowl with the nectarine puree. Mix together until all the salsa is coated. Put a layer of plastic wrap over the salsa and keep in the fridge until ready to serve. You will want the salsa to have lots of extra juices to pour over the chicken and to eat with the rice or cous cous to give it more flavor.
When you are ready to grill the chicken, you can skewer the chicken on kabob sticks or just grill the chicken breasts whole, which ever you would rather. Grill the chicken on a medium-high heat until the outside of the chicken is nice and golden brown and has nice grill marks. Turn down the grill heat to medium-low and continue to grill until chicken is no longer pink in the center and is cooked through. Serve the cooked chicken with the nectarine salsa.