Fill 4- to 5-quart Dutch oven 3/4 full with water. Bring to a boil over medium-high heat. Meanwhile, slice bottom and top off each onion, leaving outer skin on. To hollow out onions, with knife, cut an “X” through center of each onion, cutting up to but not through 2 outer rings of onion. Run knife around the inside of second outer ring to loosen middle of onion; with thumbs, push middle part of onion out. (Save insides of onions for another use.) Gently drop onions into boiling water; boil 12 to 13 minutes. (Onions will be slightly soft but retain their shape.) With slotted spoon, remove onions from water. Cool 10 minutes. Remove onion skins.