How would you season those Frisbee style mushrooms for a burger substitute?

I have 4 big mushrooms that look like cow paddies. I want to put one on an onion bun with lettuce and…what else? What kind of seasoning should I use? I don’t have any tomatoes and the grill can’t be used now. Paul Dean used that Mexican? green blender stuff mixed inside mayonnaise but I can’t remember what it’s called and she used an indoor grill. I think fried onions would be good.