Season both sides of the duck with salt and pepper. For medium-rare duck, add duck breasts to a hot pan skin side down, cook for 6-7 minutes, turning at the 4-5 minute mark. Remove and set aside to rest for at least half of the cooking time. Slice into pieces.
Heat a pot of vegetable oil to 170°C. Fry the sweet potato rounds for 5-7 minutes or until golden brown. Transfer to the frying pan that the duck was cooked in and toss in the rendered duck fat. Transfer to a plate lined with a paper towel and season with salt.
Combine all of the ingredients for the sauce and salsa in 2 separate bowls.
Butter the bread, layer lettuce, tomato slices, duck, salsa and the sauce, and top with the bread lid. Serve with sweet potato chips.