Cut the top off of the onion leaving the root side intact.
Peel the outer skin off on the onion.
Using an apple corer or knife core out the middle of the onion just as you would an apple.
Put 1/2 teaspoon of butter in each hole on top of the bullion.
Wrap in tinfoil and grill for 30-40 minutes at a temp of 350-400 degrees.
Remove from grill and top with parmesan cheese.