Special equipment: mandoline or food processor fitted with a 2-millimeter slicing blade
Prepare Potato Salad Vinaigrette: Using a mandoline or food processor fitted with a 2-millimeter slicing blade, slice potatoes across width as thinly as possible. Place in a large bowl and wash in cold, running water until water runs clear.
Bring a large pot of salted water to a boil. Add potatoes, bring back to a boil, and remove from heat. Drain in a colander. In a medium bowl, toss potatoes with red onion and reserve.
Whisk remaining ingredients in a small bowl to form a vinaigrette. Pour over potatoes and onions, and toss to combine. Cover and refrigerate up to 2 days.
Prepare Cod: Soak hickory chips in water for an hour. Meanwhile, combine brine ingredients in a medium saucepan. Bring to a boil, strain into a ceramic or glass roasting pan, and chill. When cool, marinate cod fillets about 15 minutes at room temperature.
Make a stove top smoker with that big old pot you've been meaning to throw away. (Or line a good pot with aluminum foil to keep it from aging quickly.) Place chips on bottom of pot and a rack above them. Cover pot, place on a burner over high heat. When chips start smoking, place fish on rack, cover, and cook over moderate heat 10 minutes, or until just cooked through. Transfer fish to a platter, cover with plastic wrap and chill.
To serve, line 6 serving plates with a thin layer of Potato Salad Vinaigrette. Cover each with a chilled fish fillet and serve.