Adjust the cooking rack to the bottom position and preheat your toaster oven to 425°F. While the toaster oven preheats, place the sandwich rolls inside to warm and toast.
Rub a rimmed sheet pan with 1 teaspoon of the oil.
In a large bowl, whisk together the remaining 1 teaspoon olive oil, balsamic vinegar, soy sauce, oregano, garlic powder, and pepper. Add mushrooms and stir to coat well with the sauce. Stir in bell pepper and onion. Pour vegetables onto the prepared pan.
Once the oven has preheated, remove sandwich rolls and add vegetables.
Roast vegetables, stirring occasionally, until fork tender, about 15 to 18 minutes minutes.
Carefully remove the pan and divide the veggies into two piles on the pan. Top each pile with a slice of provolone cheese. Return pan to the toaster oven and cook until the cheese is melted about 2 to 3 minutes.
Split the rolls in half, scoop the filling into each one, and dig in!