Preheat the barbecue to medium. Once hot, clean the grill.
Place the frozen duck patties on the barbecue grill and cook for 5 to 7 minutes on each side, or until the internal temperature of the patties reaches 74 °C (165 °F) in the centre.
Remove the duck patties from the grill and slice horizontally to obtain 8 thin patties. Cover each patty with a slice of cheese and keep warm.
Spread some of the creamy tomato sauce and chopped onions on the 4 bun bottoms.
Lay a patty on top of each bottom, then another bun bottom. Spread more of the creamy tomato sauce and then top with another patty. Garnish with pickle slices and chopped lettuce
Top with the bun tops and serve immediately with corn on the cob and vegetable chips.
Place all the ingredients in a bowl and mix well. Cover and keep cool until ready to serve.
