For your dressing, chop the jalapeños, then combine with the oil and vinegar in a small bowl. Season to taste with sea salt and black pepper.
Blacken the whole peppers over a direct flame on the hob, or in a griddle pan on a high heat, until charred and blistered all over. Pop the peppers into a bowl and cover with clingfilm for 10 minutes, then scrape off most of the black skin, discarding the stalks and seeds, and chop.
Grill or lightly toast the ciabatta and roughly tear into pieces.
Halve the cherry tomatoes and trim and slice the spring onions.
Combine the ciabatta, tomatoes, spring onions, prawns, celery tops and peppers in a large bowl. Drizzle with the jalapeño dressing, cover and allow to marinate for 10 minutes before serving.