Place the onion, garlic and celery in a small roasting tin or ovenproof dish with 1 tablespoon of the oil. Mix well. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes until softened.
Slice the peppers thinly and add to the roasting tin with the capers and season lightly with salt and pepper. Lay the sardines on top and drizzle with another tablespoon of the oil. Return the roasting tin to the oven for a further 15 minutes until the sardines are cooked through.
Add the mozzarella to the roasting tin and return to the oven for a final 2 minutes.
Toast the ciabatta for 1–2 minutes on both sides.
Arrange the toasted ciabatta on warmed serving plates and pile the sardines and vegetables on top. Serve drizzled with the pan juices and the remaining oil.