Finally time to assemble the burger, halve the burger buns and toast or grill them a little and start with the tomato sauce , then lettuce, then the patty, then the Mayo, then comes the sliced tomatoes and cucumbers and finally the onion rings. If you are adding a poached egg that comes last and the cheese slice always comes on top of the patty Some prefer the lettuce down and some like it in the uppermost layer.