Plain hamburger - devoid of everything except meat and bun
Plain cheeseburger - devoid of everything except meat, bun and cheese
Plain bacon cheeseburger - devoid of everything except meat, bun, cheese, and bacon

If I think there is any question that the person behind the counter doesn't understand what "plain" means, I will tell them to hold everything except the meat, bun, and cheese. That usually gets the point across