Preheat oven to 160°C. Cook pineapple slices in a pan with oil and butter for 30mins. Heat schnitzels as per instructions.
Preheat grill or salamander on high heat. Toast the cut sides of the burger buns until lightly golden. Place bun bases on plates.
In a processor blend mayo, mustard and fennel until fennel is smooth.
Spread buns with mayonnaise. Top with lettuce. Cut schnitzels, if required, and place on lettuce. Top with roasted pineapple slices, and cover with bun tops. Serve with chips.