Set a 4- or 5-quart dutch oven or braising pot oven medium heat. Add the butter and when melted, the onions, Old Bay, and pepper. Sauté onions until translucent and tender, 5 - 7 minutes. Add the carrots and celery, and cook 5 minutes, stirring often. Add the remaining vegetables and cook another 5 minutes.
Slowly add 2 cups broth, scraping up any brown bits as you go. Add the bay and thyme, bring mixture to a boil, and then turn down to a lively simmer. Cook 10 - 15 minutes, or until vegetables are tender. Taste mixture, and add sea salt as needed. Turn heat down so that broth bubbles very gently, and add the cod. Cook until cod is flaking apart and cooked through, 10 - 15 minutes. Add the crab and cream, and cook until heated through. Taste again and add additional salt and/or pepper.
While stew simmers, make the toasts. Heat a cast iron or other heavy skillet over medium-high heat. Brush both sides of the sourdough with olive oil and season with salt and pepper, and then cut each slice in half. Add as many slices as will fit to the skillet and cook 2 - 3 minutes per side, or until the edges are dark brown (almost, but not quite burnt). Flip and cook the other side. Repeat for any remaining slices.
Ladle stew into bowls and serve soup topped with fresh herbs and a couple of sourdough croutons on the side.