Combine all ingredients except chicken breasts in the blender and blend till relatively smooth. Place chicken in a 15 X 10" dish, sprinkle with salt and pepper and pour the mustard mixture over it. Turn the breasts to ensure they are completely coated. Let stand 1 hour at room temperature or 2-4 in the fridge.

Preheat the grill on high (about 400 degrees). Place the chicken on the grill and reduce the heat to low. Cover and cook 10 minutes on the first side. Check that breasts are not burning and reposition on grill if necessary. Turn off heat so only 1 burning is left on low. Flip chicken and cook another 10-15 minutes. Cut into the breast to check for doneness.

When opaque but still juicy remove the breasts to a platter and cover with foil for 5 minutes. Serve breasts whole or sliced according to your preference.