Rinse cod well under cold running water. Cut into five or six pieces and place in a large bowl. Cover with cold water and refrigerate for at least 24 hours, changing the water several times, until fish is softened and there is no trace of salt.
In a skillet, heat oil over medium heat. Add onions, carrots and celery and cook, stirring, until softened. Add garlic, rosemary, salt and peppercorns and cook, stirring, for 1 minute. Transfer mixture to slow cooker stoneware. Add cooked beans, vegetable stock or chicken stock and white wine and stir well. Cover and cook on Low for 8 hours or on High for 4 hours, until vegetables are tender.
Drain cod. In a heavy pot with a lid, bring 4 cups water to a boil. Add lemon juice and bay leaf. Add cod and cook until water almost reaches a simmer (it should not boil). Cover and remove from heat. Let fish stand for 15 minutes, until it flakes easily. Using a slotted spoon, lift out cod. Remove any skin, bones or dark pieces and flake cod into small pieces.
Add cod and cream to slow cooker. Cover and cook on High for 1 hour, until mixture is hot and bubbling. Serve piping hot. Serves 6 to 8.
TIP: Salt cod is easy to cook, although it requires a long period of soaking with several changes of water. Planning ahead and cooking the dried beans in conjunction with soaking the fish can make this a relatively time efficient recipe.
MAKE AHEAD: You can soak and cook the beans for this recipe while the cod is de-salting. Soak the beans for at least 8 hours, then cook overnight in the slow cooker. In the morning, continue with step 2.