These are a very tasty change-up from grilling a hamburger. What you will need: 4 portabella mushrooms; large sweet onion; beefsteak tomato; swiss cheese; arugula; spinach; original yakitori sauce; ciabatta rolls; olive oil or cooking spray.
Here is what we did. Take a damp paper towel and gently clean the portbellas and remove the stem but leave the fins. Place on a sheet of wax paper underside facing up and drizzle a small amount of original yakitori sauce into each mushroom cap, about one to two tablespoons and let sit for a half hour. Helpful hint: (we cleaned and used the trays they came in to hold them while they marinated in the yakitori sauce, it also made it convenient to carry them to the grill).
Fire up your grill on medium heat and oil or spray to prevent sticking. Place the portabellas smooth side down for 5-7 minutes (we did 7 min.). After 7 minutes flip and lightly baste the tops with some more yakitori sauce. At this time we also placed our sweet onions, cut about a quarter inch thick, onto the grill. After 3-4 minutes we flipped the onions and with a minute left we placed the swiss cheese on each mushroom cap. Remove from grill and place each on a sliced ciabatta roll. Top with tomato, spinach and arugula. You definitely need a heavier roll to stand up to the juiciness of the portabellas. Enjoy!