Heat oven to 375 degrees F.
Under cold running water, wash chickens inside and out; pat dry with a paper towel. Generously season inside and out with salt and pepper.
Roughly chop 2 sprigs rosemary. In a medium bowl, mix together honey, butter, chopped rosemary, lemon juice, and lemon zest. Set aside.
Combine lemon halves, remaining 4 sprigs rosemary, garlic, and onion; divide among 4 birds to fill cavities. Using a pastry brush, coat outside of birds with lemon-honey glaze. Place birds in a large roasting pan on rack positioned in the middle of the oven. Baste every 15 minutes, and brush with any remaining glaze. Roast until an instant-read thermometer inserted into the deepest part of the breast registers 180 degrees F and juices run clear when pierced with a knife, about 1 hour. Let rest for 10 minutes before carving. Serve hot or at room temperature.
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