Clean the leeks: Place sliced leek rounds (white and light green parts only) into a large bowl, then fill bowl with cold water until leeks are covered. Allow to sit for at least 5 minutes so that the grit from the leeks falls to the bottom of the bowl. Remove leeks from bowl, place on top of a towel and pat dry. Discard water.
Place a large, heavy pot on the stove, turn heat to medium and drizzle in olive oil.
Add leeks, carrots, celery, garlic, salt, pepper, thyme and parsley, and cook, stirring occasionally, until the onions are soft and fragrant, about 10 minutes.
Add bay leaf, beans and water and bring to a boil.
Lower heat to a simmer, cover partially with a pot lid and cook, stirring occasionally, for about 30 minutes or until vegetables are tender and water is mostly absorbed.
To make the breadcrumbs, preheat your oven to 350 degrees.
Place the breadcrumbs on a sheet pan lined with foil or parchment paper, add olive oil, salt and pepper, then evenly distribute the breadcrumbs on the pan.
Place in oven and cook until the breadcrumbs are golden brown and fragrant, about 10 minutes.
Remove from oven and allow to cool, then toss with chopped fresh parsley.
Remove bay leaf from bean mixture, and use a fork or potato masher to mash some of the beans.
You can leave the beans in their pot and move directly to the next step, or, like I did for this photo, spread the beans evenly in a casserole dish, then top with breadcrumbs.
Sprinkle breadcrumbs over the bean mixture and serve.
