Prepare the charcoal for the grill.
Season the corn with salt and pepper. Place the corn on the grill and cook for about 1 hour.
Season the garlic, onions and tomatoes with olive oil, salt and pepper.
Wrap the garlic in foil and place on the grill. Cook for 45 minutes or until the garlic is tender. Remove from the grill and squeeze out the garlic cloves. Place the garlic in a mixing bowl, and smash the garlic with a fork.
Whisk in the melted butter. Pour the melted butter mixture into a tall slender glass, (a glass the length of the corn).
Place the onions and tomatoes on the grill and cook for 4 to 6 minutes on each side.
Season the avocado slices with salt and pepper.
Remove all the vegetables from the grill and place on a large platter with the avocado slices.