Soak the beans, uncovered, in a large bowl of cold water on the counter for at least 4 hours or up to overnight.

Drain and set aside.

Arrange a rack in the lower third of your oven and preheat to *F.

Trim the leeks, discarding the tough green tops, halve vertically and rinse well in cold water, making sure to clean out any dirt trapped between the layers. Slice into thin half circles.

In a large Dutch oven, or ovenproof casserole over medium heat, warm the olive oil and butter. Add the celery, garlic and leeks and cook until the vegetables are softened, -5 minutes.

Add the beans, herbs, red pepper flakes to taste, 1/4 tsp salt and a generous amount of black pepper.

Stir in the vegetable broth and 1/2 cup water and bring the mixture back up to a gentle boil.

Cover the pot with an ovenproof lid or cover it tightly with foil. Place in the oven and cook, checking occasionally to make sure the pot is never dry, until the beans are soft throughout but not falling apart, 3 to 3 1/2 hours.

If the pot seems dry, add water in 1/2 cup increments and stir once or twice more.

Taste and adjust the salt and pepper if necessary. (Again, my beans seemed fully cooked after 80 minutes, then I added the bacon and removed the lid because I had lots of liquid. I added the cheese after almost two hours.)

Remove pot from oven and turn the heat up to 500*F.

Sprinkle the mozzarella and Parmesan on top of the bean mixture and put back in oven, leaving the lid off.

Cook until cheese is completely melted and brown in spots, 8 to 10 minutes.

Serve hot.