Preheat oven to 500 degrees (broil). In a large skillet place oil, onion, bell pepper, mushrooms over medium heat. Continuely move around veggies making sure not burn. Cook for around 5-7 minutes. Slice hoagie rolls in half and put butter on each side. Place on a cookie sheet and broil for about 2 minutes or until lightly brown. Place the mayo and garlic in a bowl and mix together. After you take the rolls out of the oven spread the mayo on each side of the roll. then place the roast beef on one side with a slice of cheese that has been cut in half. On the other side place the veggies with another slice of cheese that has been cut in half. Place in the oven to broil for about 3 minutes or until the cheese is melted. Fold the sides together and ENJOY!