Soak the beans in plenty of water overnight. The next day drain, rinse and drain again.
Preheat the oven to 180°C. Chop the celery and onions, thinly slice the garlic and cut pancetta cut into cubes.
Put all ingredients in a large, heat-resistant, lidded dish then cook for 2 hours in the oven under cover, stirring occasionally. When the beans are soft, season to taste with salt and pepper.
Meanwhile, tear the ciabatta slices into bite-sized chunks, arrange them on a baking sheet, drizzle with a little oil and sprinkle with coarse sea salt and the chilli flakes. Bake for 10-15 minutes until golden.
Serve the soup with the toasted ciabatta pieces.