Drain your soaked cannellini beans, and add to a pot with fresh cold water to cover. Bring to a boil and boil for an hour to an hour and a half until beans are soft. Top up with water as necessary.
Preheat oven to 200C
In a large frying pan over medium heat, saute the the leeks and celery in the olive oil and butter until soft.
Add the minced garlic and saute a minute or two more.
Add the cooked white beans (or two tins of organic bean of choice, rinsed), cream, stock, salt, pepper, and herbs. Bring briefly to a boil.
Turn the heat down and stir gently until you can see the sauce thickening (cooked dried beans will have enough starch to do this, if you are using canned beans, thicken the sauce with a little cornflour).
Add half the parmesan and stir to combine.
Test for seasoning, and adjust as necessary.
Pour into an oven-safe casserole dish and top with the rest of the parmesan.
Bake for 20 minutes until the edges are bubbling, the top is browned, and the casserole is heated through.