In a large bowl filled with cold water, soak cod for 4 days, changing the water twice per day. Remove cod from water. Pull off any skin, and discard. Using kitchen pliers or tweezers, remove any bones from the spine.
In a large saucepan, combine milk and 6 cups water. Add onion, celery, and garlic; season with salt. Bring mixture to a boil. Reduce heat, and simmer for 20 minutes. Remove from heat, and transfer cod to the bowl of an electric mixer fitted with the whisk attachment.
Add just enough of the cooking liquid to moisten the fish, 1/4 to 1/2 cup. Be sure not to add the garlic clove. Beat cod on low speed. When it breaks into small pieces, increase the speed, and begin adding olive oil in a thin, steady stream. The quantity of oil used depends on the quality and fat content of the cod; stop adding oil when the mixture is very creamy and emulsified. Stir in parsley, and season with salt and pepper.
Using an oval ice cream scoop, neatly place 3 mounds of the cod mixture on each of 6 plates. Garnish with a handful of radicchio and Boston lettuce. Drizzle with oil and vinegar. Serve with polenta on the side.