I will make this at work every now and then, usually as a treat for one of the folks over in the Pro Shop.
I like to use a mix of mushrooms, sliced button, shiitake, and portabello. I saute in clarified butter, with a pinch of salt and pepper. When the mushrooms just start to caramelize (this is on a commercial range, so I get much higher BTU output), I'll hit it with about 2 T of Au Jus, and cook that down. Mushrooms on top of the burger, then the cheese, and a quick shot into the salamander to melt the cheese.
When it comes to cooking the burgers, I'm a salt-and-pepper person, unless there is some Montreal Steak Seasoning handy. I'm also a flame-broiled burger person.