Toss the de-stemmed mushrooms into a large mixing bowl with olive oil and salt and stir to coat.Arrange the mushrooms top-down on a sheet pan that has been sprayed with cooking spray. Roast them in a 400 degree preheated oven for 12 minutes. When they are cool enough to handle, toss out any water pooling inside the mushrooms then place them back on the pan, top-down.Fill the caps with the minced pepperoni, then spoon a thin layer of marinara over the top. Top the filled mushrooms with a pinch of the shredded mozzarella.While they are roasting, stir together the parmesan and parsley in a small bowl.Place them back into the oven to melt the cheese for 8 minutes, then remove from the oven and sprinkle with the parmesan and parsley, then place them back into the oven for just one minute more. Serve hot or at room temperature.