Heat the butter in a large soup pot over medium heat, add onions and cook, stirring, until translucent. Add the potatoes, salt, and pepper and enough water to cover. Bring to a boil, lower the heat, and simmer for 5 minutes. Add the cod and enough water to cover. Simmer for 5 minutes more. Let the chowder cool. Stir in the half-and-half and milk (add more milk if necessary). Refrigerate the chowder, covered, overnight.
When ready to serve, reheat the chowder in a double boiler or slow cooker, stirring only occasionally--you don't want to break up the cod too much. Serve with oyster crackers and enjoy!