Make the brine in a large stock pot. Add 4 cups hot water with kosher salt. Whisk together until salt dissolves.
Add remaining water to the pan. Squeeze the lemon juice into the brine and toss the rinds in with rosemary and peppercorns. Add ice and stir until mixture is cooled down.
Submerge chickens in the brine. Cover and refrigerate up to 4 hours.
Drain Chickens and discard brine.
Prepare a grill fire to 350° using hickory or pecan chunks for smoke flavor.
In a small skillet, melt butter over low heat. Add Worcestershire sauce, dijon mustard, onion flakes, garlic and pepper. Whisk together and remove from heat to cool slightly.
Place chicken on a baking sheet and pat dry with paper towels. Carefully loosen the skin around the breast.
Spoon butter sauce under the skin, rubbing it around to coat well. Coat the outside of the chickens with remaining butter, rubbing all over.
Truss the chicken with butcher twine.
Grill chickens over indirect fire with the grill closed for about 1 hour 15 minutes.
Check the internal temperature of the chickens, using a digital meat thermometer, in the thickest part of the thigh near the bone. Continue cooking until chicken reaches 165°.
Remove to a clean sheet pan and rest 15 minutes before carving.