Preheat the oven to 400 degrees F.
In a medium bowl mix together the roasted red bell peppers, olives, cheese, scallions, olive oil, salt, and pepper.
On a parchment paper-lined baking sheet place the mushrooms, cavity side up. Spoon the filling into the cavity of the mushrooms. Bake until the mushrooms are tender, about 20 minutes.
Transfer the mushrooms to a serving platter, sprinkle with chopped basil, and serve.