Melt the butter in a saucepan over medium-low heat. Whisk in the flour, salt, pepper and celery seed; cook and stir for 1 minute. Gradually whisk the milk into the flour mixture and bring to a simmer over medium heat. Cook and stir until the mixture is thick and bubbly, 5 to 10 minutes.
Stir in the potatoes, onion and capsicum. Pour the potato mixture onto a large sheet of aluminium foil. Sprinkle with paprika. Fold the foil loosely around the potato mixture and seal tightly. Grill on covered barbecue turning the packet every 15 minutes until the potatoes are tender, about 45 to 50 minutes.
Tried this the other night as now living in motorhome and doing a lot of cooking on Council Barbecues. Chopped potatoes and carrots in to cubes added to cooked sauce and tumbled the lot into aluminium packet,cooked slowly on barbecue plate....yummo - 31 Jan 2013