Combine diced potatoes and onion onto a piece of heavy duty foil - shiny side up - large enough to leave 2-3 inches of clear foil on either side and 4-5 inches top and bottom of your potato pile
Sprinkle 1 tsp of the steak spice over potato mixture, toss, then sprinkle the 2nd tsp.
Place chunks of the butter both on the bottom and top of your potato pile
Bring the top and bottom edges of the foil together and fold over several times, leaving some air space between potatoes and foil.
Repeat with each side of your foil package until a sealed pouch is formed
Cook pouch indirectly on the BBQ - meaning on an upper rack, over a non-burning element, or to the side of your burning charcoal - for 30mins, turn the pouch every 8-10. Total cooking time will vary with heat level of BBQ/fire mins
When potatoes are cooked, the pouch will swell up with steam. Note - this will only work if your pouch is well sealed and you haven't accidentally poked a hole in it during handling
Open foil pouch and serve. Watch out for the steam when you first tear the foil - especially if the pouch is puffed up!