Onion measurement missing from recipe

That seems to be information missing from the recipe, but I think you could easily adjust to suit yourself. I'd do no more than a half cup of onion, small chopped. But if you are a major onion fan, you could certainly do more- and if you don't like it at all, you could leave it out. After all, you do personalize a cheeseburger to suit your tastes. If you have an onion hater in the family, you can also sometimes use a sprinkling of onion powder. Funny how people who do not like bits of onion will love the taste if you use onion powder. Anyway, add or subtract onion to suit your taste. It's not that critical to the recipe success.