How do you like to make them? I know a lot of people who just want a plain burger with a slice of cheese and some condiments and other people like to mix things with the meat. I'm the later, but usually just make them plain when cooking for others.
When I?m by myself or with someone who doesn't care I'll always mix in mushroom and onion. I've also mixed in jalapeno, Worchester sauce, or ranch dip with that.
Last night I mixed in jalapeno and wrapped a thick slice of bacon around it. I fixed some more bacon to put on top and then saut?ed mushroom, onion, and orange pepper in the bacon grease. It went bacon wrapped jalapeno burger, Swiss cheese, bacon, veggies, more Swiss cheese, and more bacon again. It was delicious.
I have a friend who just told me about mixing in an egg yolk. I think I'm going to try that next time.