Soak the dried porcini mushrooms in the boiling water for 25 minutes.
Once softened, strain the porcini mushrooms from the liquid and set aside.
Heat a heavy-bottomed, oven-safe pan to medium heat. Add the olive oil and butter to pan.
Saute the onions, carrots, and celery for 6 to 8 minutes, until softened.
Add the cremini mushrooms, oyster mushrooms, and garlic to the pan. Saute for 4 to 5 more minutes.
Add the porcini mushrooms, fresh thyme, dried oregano, bay leaves, tomato paste, Marsala wine, mushroom stock, white beans, salt, and pepper to the pan. Mix well and simmer for 5 to 10 minutes.
Meanwhile, combine the breadcrumbs, grated Parmesan, thyme, and olive oil to create the breadcrumb topping.
Top the mushroom cassoulet with the breadcrumb mixture.
Transfer the pan to the oven and bake for 30 to 40 minutes, until the top is golden and the cassoulet is bubbling. Serve hot.