This is a good mix of flavours and textures. The sweetness of the leeks goes well with earthy beans and salty crisp pancetta. Celery gives a bit of crunch and greenness, and the green peppercorns leave a mild smoky aftertaste.
If using dried beans, drain the soaking water and put the beans in a saucepan with fresh water to cover. Bring to the boil, boil for 15 minutes, then simmer for about 30 minutes more until the beans are just tender but not disintegrating. Drain, reserving the liquid. (If using canned beans, drain and rinse thoroughly.)
Melt the butter in a large saucepan. Add the leeks, celery, bay leaf