Heat olive oil in a medium skillet over medium to high heat and add chopped onion. Cook until soft and slightly browned, stirring frequently, about 4-5 minutes. Remove from heat and set aside to cool. In that same pan cook the red pepper, red onion and mushrooms until soft and slightly brown, 5-6 minutes reserve until ready to assemble the subs.
In a large bowl, combine the ground turkey, bread crumbs, parsley, egg, black pepper and oregano. Add the cooked, cooled chopped onions and mix all the ingredients well. With wet hands, make 24 meatballs, about 1/4 cup each, place on a baking sheet and put in the fridge until sauce is ready.
In a large sauce pot, combine the tomatoes, garlic, onion and oregano. Bring to a simmer and cook for 10 minutes. Add to the sauce the chipotle peppers and simmer for another 2 minutes. Remove the meatballs from the fridge and add one at a time to the sauce. Cook for about 10 minutes on medium heat, or until cooked through.
Split and hollow out the rolls. Toast them for 2 minutes and remove from oven. Begin by placing 3-4 meatballs, depending on the size, on the bottom half of the roll. Layer with some of the pepper, onion, mushroom mixture, followed by some cheese and pop back in the oven to melt the cheese for 3 minutes. Top with the remaining half of the bun and serve!