Preheat an outdoor grill for medium heat, and lightly oil grate.
Peel each onion, and core out the center. Drop a beef bouillon in the core of each onion, and cover the bouillon with 1 tablespoon of butter. Wrap tightly with aluminum foil.
Cook on grill for 30 to 60 minutes, or to desired doneness. Be careful not to puncture the aluminum foil, or the juices will drip out, and the onion will burn.
