Prepare and wash mushroom cap. Clean it well and fully dry.
Take out the stem of the mushroom, and with a spoon or hands clean all of the dark gills underneath. Lightly coat with olive oil and season with salt and pepper, to taste.
Place the mushroom cap on the grill, cap part down.
While the cap is cooking, prepare your bun and toppings. Toast your bun/roll, add pesto spread, and all of the toppings that you would like (salad/lettuce, tomato, cheese, etc.)
Once the cap is soft, remove from grill and place on bun. Enjoy!