Lightly wipe dirt off the mushrooms using a damp paper towel. Gently remove the stems from the mushrooms and scrape out the gills using a spoon.
Brush both sides of the mushrooms with olive oil and sprinkle with ½ teaspoon salt. Place mushrooms, gill side up, on a baking sheet. Bake until tender, flipping once, about 8 minutes per side.
Meanwhile, form the meat into four equal sized patties and season both sides with 1 teaspoon salt and fresh ground pepper.
Spray or lightly oil a cast iron skillet and place over medium heat. Add the patties and cook about 2-3 minutes per side for medium rare. Place a piece of cheese on each patty, cover and cook until cheese melts, about 1 minute. Transfer patties to a plate.
Place the patties on top of 4 mushroom caps. Add garnishes of your choice and then top with remaining mushroom caps. Sprinkle buns with sesame seeds, if desired.
Recipe by Three Hungry Boys at https://www.threehungryboys.com/2017/05/27/portabello-mushroom-burgers/