Nothing goes better with sautéed mushrooms than melty gooey Swiss cheese and a pile of sweet caramelized onions. Put all of that on top of a perfectly grilled burger and you've got yourself a Sauteed Mushroom & Swiss Burger with Caramelized Onions!
Heat grill to medium-high.
Divide hamburger in to eight equal balls. Flatten and season both sides.
Grill to desired doneness. Remove from heat.
Caramelize onions (see link below for recipe). Saute mushrooms with butter until tender; set aside.
Once burger is done, top with onions, mushrooms, cheese, & lettuce.
Here's a great tutorial on how to caramelize onions: http://www.simplyrecipes.com/recipes/how_to_caramelize_onions/