Grease a small dessert bowl, ramekin or disposable cup with butter.

Break egg into a bowl and pierce yolk with a fork. (Piercing is necessary to prevent "exploding" during cooking.). Sprinkle little, salt, pepper and oregano on top of the egg.

Cook it in a microwave for 90 seconds. Demold it on a plate.

Spread 1 tbsp burger mayonnaise on each half of burger bun.

On bottom, half of burger bun place cooked egg, onion slices, add a dollop of mayo and top it up with other half of the bun. Serve