Soak the dried porcini mushrooms in the boiling water until soft, about 20 minutes. Strain the mushrooms, reserving the liquid. Finely chop the porcinis and set aside.
While the porcinis soak, preheat the oven to 375° F/190° C.
Toss the the baby portabella mushrooms with 3 tablespoons of the olive oil and a sprinkle of kosher salt and black pepper. Arrange on the prepared baking sheet and place the rosemary and thyme sprigs on top of the mushrooms. Roast until the mushrooms begin to release their liquid, then drain off the liquid and combine with the porcini broth. Return the mushrooms to the oven and continue roasting until brown and tender, about 20 to 30 minutes. Discard the herbs. Chop about 1 cup of the mushroom quarters and set aside.
Warm the remaining two tablespoons of olive oil and sauté the onions, carrots, and celery until soft. Add the porcinis, quartered baby portabellas, cooked cannellini beans, and mushroom broth. Bring to a boil, then reduce heat and cook covered until the beans are soft, about 20 minutes. Stir in the milk and parsley and allow it to warm. Season with additional kosher salt and freshly ground pepper to taste. Puree in the blender, dividing into batches as necessary.
Serve with some of the chopped roasted mushrooms on top.