- Preheat the oven to 190C/375F.
- Halve the peppers, keeping the stalks on.
- Remove the white seeds and pith.
- Place the peppers, cut side up, in a shallow roasting tin.
- Scatter over the onion and garlic, season with salt and pepper and drizzle over half the olive oil.
- Roast for 40 minutes until the peppers are tender.
- Leave to cool.
- Arrange the cold peppers on a serving plate and pour over any juices left in the roasting tin.
- Scatter over the olives, mozzarella pearls and basil.
- Whisk together the remaining olive oil with the balsamic vinegar and pour over the peppers.
- Cover and leave to marinate in the refrigerator for at least 2 hours or overnight before serving.