BBQ Roasted Mushrooms

Had these on a recent fishing trip - fantastic.  This is the general recipe, about 1 medium sized onion to 16 oz of mushrooms works well - scale as needed.

Mushrooms (cut in large chucks) (Crimini are quite good, white mushrooms will also work nicely)
Onion (chopped to bite size portions)
Montreal Steak Spice (a fairly liberal covering)
Pepper (a few shakes)
Butter or Margarine (1 heaping tablespoon)

Mix ingredients and place in an aluminum tray, cover tightly with aluminum foil and roast over medium heat on the BBQ for about 45 minutes.
Roasted mushrooms etc.