Preheat the grill. Arrange the peppers on a baking sheet with the red onion. Sprinkle with the olive oil, then grill until soft and lightly browned - about 6-8 minutes.
Look out for 'everyday' ranges of peppers, which are more economical to buy.
Toast the slices of bread and share the pepper mixture between them. Arrange the mozzarella cheese on top, then return to the grill for 1-2 minutes, until the cheese begins to melt. Serve with mixed salad leaves.
The pepper, onion and mozzarella mixture makes a delicious filling for warmed wholemeal pitta breads.