There are few techniques that might help. First of all you have these pre-rendered phrases that you are probably repeating in your head and you already beleive you will not be able to get them across fluently. And you are correct that belief reinforces the situation and you stutter. So try and speak as improvisational as possible. Maybe change things to "So do you want that without onions"? Or make a game out of it and see how many different ways you can say the same thing differently, but completely spontaneously. Another technique that can be effective is whispering. Obviously not the entire sentence just the start, to get things going. Or even drop the first letter. Saying "ould you like onions with that" and whispering both might for a second be odd for a customer If they even happen to notice. But not nearly as much as you locking up for a minute unable to speak or breathe.