Preheat the oven to 200°C/gas mark 6.
1) In a medium bowl mix together the roasted red peppers, olives, cheese, spring onions, olive oil, salt, and pepper.
2) On a greaseproof paper-lined baking tray place the mushrooms, cavity side up. Spoon the filling into the cavity of the mushrooms. Bake until the mushrooms are tender, about 20 minutes.
3) Transfer the mushrooms to a serving platter, sprinkle with chopped basil, and serve.