For an extra layer of flavor, consider adding a pinch of smoked paprika or garlic powder to the mushroom mixture. If you prefer a spicier popper, leave some of the seeds in the jalapeños. These poppers can be assembled ahead of time and baked just before serving. Store assembled, unbaked poppers in the refrigerator for up to 24 hours. For a crispier topping, sprinkle the cheese and broil for a shorter amount of time.