Soak 10 (10-inch) wooden skewers in water; set aside. Bring a large pot of salted water to a rolling boil. Add the onions, and return to the boil. Cook until just tender, about 5 minutes. Remove the onions to a bowl of ice water to stop the cooking process.
Remove the skewers from the water. Skewer the onions horizontally (not through the root end), fitting about 8 onions on each skewer.
Place on the heated grill and cook over low heat until brown and caramelized, about 30 minutes.