Wipe the mushroom with a damp cloth or paper towel. Remove and keep the stem and gills.
Lightly brush mushroom, inside and out, with olive oil:

       4 - 4" portabella mushrooms

Saute and set aside:
     
       2 teaspoons butter
       Dice mushroom stems into 1/4" pieces.
       1 onion, sliced in rings, separated
       1 bell pepper, thinly sliced lengthwise
        the mushroom gills

Lightly butter and grill. Set aside:

       4 potato-bread hamburger buns

Using medium heat, barbeque the mushrooms with the rounded top-side facing UP until mushroom feels hot. About 5 minutes.

Turn mushroom over on the grill, fill and cover with:

       Fill hollow side with a portion of sauteed onion mix.
       1 slice swiss cheese on each mushroom over onion mix

BBQ with barbeque lid closed until cheese melts.

Serve with normal hamburger condoments.

Servings: 4