Is the profit margin so low that its necessary to reserve food from other orders? Why would a plain burger have old crusted mayo and smell like onions? Why would the same burger only be hot on the bottom and looks grilled?

Rather just put it out there and spend money else where instead of a burger spit on

I agree something's wrong so you may want to talk to the manager

That's a question to ask the manager on duty. Maybe they could have replaced it.