Make the hens: About 2 hours before serving, preheat the oven to 400 degrees. Cover the bottom of a large shallow baking pan with the onion rounds. Tie together the legs of each hen with cotton string. Arrange the hens, breast side up, on top of the onion rounds, leaving some space around each bird so the skin will crisp. Season with salt and pepper.
Make the glaze: Mix all the glaze ingredients (except salt and pepper) together in a small bowl.Spoon about 2 teaspoons of the mixture over each hen.
Place the pan of hens in the preheated oven and reduce the heat to 325 degrees. Roast the hens for 30 minutes, basting with their pan juices once or twice. Then raise the heat back to 400 degrees, baste each hen with about 2 tablespoons of sauce, and continue roasting and basting with pan juices for 20 more minutes, or unril the thickest part of the breast reads 160 degrees on an instant-read thermometer. At this point spoon a bit more sauce over the hens and run them under the broiler briefly, so they turn a deep, rich brown. Watch them carefully so they don't burn.
Remove the hens from the pan to a platter and let them rest in a warm placefor 10 to 15 minutes. Set the hens' roasting pan on two burners and remove the onions from the pan. Add any remaining basting sauce/glaze to the pan and raise the heat to medium-high. Using a wooden spatula, scrape any brown bits sticking to the pan and stir as you boil the pan sauce for 3 minutes, or until it's rich tasting and almost syrupy. Taste and adjust for seasoning.
To serve the hens, place them on a warm platter, scrape the reduced pan juices over them, and scatter with the optional pine nuts. Serve hot.