Heat butter in a griddle pan on medium flame.
Grill the rings of pineapple and onion until caramelised and there are nice grill marks on both sides.
Spread one tablespoon mayonnaise on each half of burger bun.
On the bottom half of the bun, place lettuce leaf, followed by burger patty, onion rings, pineapple (a dollop of mayonnaise on pineapple) and top it up with other half of the bun. Serve.