Preheat the oven to 400F degrees. (I use my toaster oven.)
Cut the baguette in half lengthwise. Place the halves on a baking sheet, insides facing up. Bake for 5 to 7 minutes, until lightly toasted.
While the bread is toasting, lightly coat a nonstick skillet with spray and place it over medium heat. Add the onions, peppers and mushrooms and cook, stirring occasionally until just tender, but not browned, about 5 minutes.
Top each toasted baguette evenly with half the marinara sauce, then half of the vegetable mixture, then the cheese. Sprinkle evenly with oregano and crushed red pepper flakes, if using. Place the pepperoni over that, followed by the remaining vegetable mixture.
Return the pizza to the oven and bake until the cheese is just melted and the bread is crisp, about 5 to 8 minutes. Serve immediately.
