Place oil in a skillet, on heat over medium-high heat. Once hot, add the mushrooms, and sauté until they have released their water, and are starting to brown. Add in the chopped peppers, and continue to cook until veggies are tender.
While veggies cook, cut open the hoagie roll, being careful not to cut all the way through. Remove a little of the bread inside the hoagie roll so the filling has somewhere to sit. Microwave the Alfredo sauce until hot.
Once the veggies are cooked, place them on the hoagie roll, and top with some of the hot Alfredo sauce (you’ll want to reserve some sauce for topping the sandwich).
Pop the sandwich under the broiler for 2-5 minutes, or until the bread starts to toast and the sandwich is nice and hot.
Carefully remove the sammie from the broiler, and top with the shredded lettuce, avocado, and tomato slices, and additional Alfredo sauce if desired.