Soak corn in water to cover at least 30 minutes or up to 4 hours.
Remove 3 or 4 pieces of husk from 1 or 2 corn ears; tear these pieces lengthwise into 1/4-inch-wide strips (you'll need at least 16 to 20 strips).
Pull back but don't remove the husk on each ear. Discard corn silk.
Brush each ear lightly with butter and sprinkle with 1 tablespoon cheese. Pull husk up on 1 side of each ear and set husk-covered side on a flat surface. Arrange equal portions of onion slices on corn; pull remaining husk up to enclose onion and corn. Tie tip end of each ear with a husk strip (knot strips if pieces are too short). Tie another husk strip around center of each ear.
Lay corn on a barbecue grill over a solid bed of hot coals or high heat on a gas grill (you can hold your hand at grill level only 2 to 3 seconds). Close lid on gas grill. Cook, turning often, until corn is hot, about 20 minutes.
Pull open husks and season corn with salt and pepper to taste.
