Ask your butcher or game processor to remove the fat to reduce the gamy taste. Also ask to have the silver skin or white muscle fiber removed.
For the marinade, mix all wet and dry ingredients until blended. Add the onions and stir. Place the filets into a shallow non reactive dish. Pour the marinade onto the filets, cover and refrigerate for 72 hours, turn the meat over once a day.
Remove the filets from the dish and place on paper towels to dry off the marinade before you light the grill. Reserve the remaining marinade for a basting sauce. Place charcoal in a pyramid shape and light with either lighter fluid, electric starter or chimney starter. Wait until coals are gray and spread in a single layer for the direct heat method. Spray grid with non-stick spray and let grid heat over the hot coals (the hotter the grid, the less chance the food will stick). Place filets on the grid. Grill for 6-7 minutes, baste with the marinade and turn, grill 5-6 more minutes, baste again with the marinade until it is cooked to medium rare. The filets cook very quickly, the cooking time should be less than 13 minutes depending on if you like them rare or medium rare.
Venison is very lean to begin with so basting is a must in this instance. It will not have the maximum flavor if cooked to the medium well or well done stage.
Suggested Wine: Von Stiehl Dry Cherry