I generally like to use 93% lean beef for this recipe and it tastes great.
You can use canned sliced mushrooms, or if you prefer, you can saute some fresh mushrooms lightly prior to adding it to the cheese/mayo mixture.
I usually use Canola oil mayo, which has half the calories of regular mayo and tastes great.
If you don't have a grill, you can cook these on the stovetop over medium high heat for about 4 minutes on each side.