If you can't find boxed salt cod at your local fish market, try GortonsFreshSeafood.com. The fish will need to be soaked before preparing the dish, so start this recipe 1 to 2 days in advance.
This recipe is from the Stirring Up Memories Christmas Menu.
Preheat oven to 400°. Evenly pierce baking potato and place on baking sheet. Bake 40 minutes, or until fork-tender and cooked through. Immediately split open to allow steam to escape. Set aside to cool. Reduce oven temperature to 350°.
Meanwhile, place red potatoes in a steamer basket set in a large saucepan filled with 2 inches water. Bring water to a boil, cover and steam potatoes 20 minutes, or until tender when pierced. Remove pan from heat and drain; set aside and cover to keep warm.
In an ovenproof skillet, combine milk, parsley, celery sprigs, bay leaf, and peppercorns. Bring mixture to a simmer and immediately remove from heat. Arrange cod in skillet in a single layer; cover tightly with foil. Bake 10 to 12 minutes, or until fish just flakes with fork. Transfer to a cutting board, cover with foil, and set aside until cool enough to handle.
Meanwhile, peel baked potato. In the top of a double boiler, or in a large bowl set atop a pot filled with simmering water (bowl should not touch water), mash or rice potato until smooth. Remove bones from cod; discard. Using a sharp knife, chop cod against the grain. Add to potato in double boiler.
Smash garlic cloves with flat side of knife. Top with salt and mash together until paste forms. Add to double boiler, along with cream. Using an electric hand mixer, beat cod mixture at medium speed. Adding oil in a steady stream, continue whipping until creamy, about 2 minutes. Season to taste with pepper and lemon juice (if using). Cover to keep warm, stirring occasionally.
To serve, slice steamed potatoes into 1/2-inch-thick rounds. Spoon a small mound of whipped salt cod onto each potato round, top with a small dab of butter (if using) and caviar, and sprinkle with chives or red onion. Serve warm.