Light a grill or preheat a cast-iron grill pan. In a blender or food processor, combine the cilantro, mint, garlic, jalapeño and sugar and pulse until chopped. Add the water, lime juice and 1 1/2 tablespoons of the grapeseed oil and process until fairly smooth; season with salt and pepper.
Lightly pound the breasts to an even 1/2-inch thickness. Brush the chicken with the remaining 1/2 tablespoon of grapeseed oil and season with salt and pepper. Grill over a moderately hot fire, turning once, until lightly charred and cooked through, about 6 minutes. Transfer the chicken to plates, spoon some of the dressing on top and serve, passing extra dressing on the side.
Serve With: Couscous or rice.
Notes: One serving 239 calories, 9 gm total fat, 0 gm saturated fat, 6 gm carb.