Preheat an outdoor grill to medium heat. Hubby used his gas grill, but any kind would work fine. (My mom actually uses a plastic container to cook hers in the microwave.)
Peel brown outer layers from onions. Using a sharp knife, cut a cone-shaped core from the top of each onion, about 2/3 of the way into the vegetable. The core should be about 1 1/2 inches wide at the top. Insert a bouillon cube into the hollowed-out space in each onion.
Top each with 1 teaspoon of margarine. Wrap each filled onion in a single sheet of aluminum foil, leaving a small vent at the top of each.
Set the foil-wrapped onions on the preheated grill vent-sides up, and grill until onions are tender, 20 to 30 minutes.