How to Make It
In a medium bowl, combine the celery root, carrot and parsnip. Add the vinegar and parsley, season with salt and pepper and toss well.
Preheat the oven to 400°. Roast the potatoes for about 35 minutes, or until tender. Let cool slightly, then peel the potatoes and pass them through a ricer or coarse sieve. In a bowl, combine the potatoes, smoked fish, eggs, butter, chives, parsley, salt and pepper. Shape the mixture into 16 patties.
Lower the oven temperature to 300°. Spread the bread crumbs on a plate. Dredge the fish cakes in the bread crumbs and tap off any excess.
In a large skillet, heat 1/4 inch of canola oil until almost smoking. Fry the fish cakes in batches over moderately high heat until browned and crisp, about 3 minutes. Turn the fish cakes, lower the heat to moderate and cook until browned on the bottom, about 2 minutes longer. Transfer to a plate and keep warm in the oven while you fry the rest of the cakes.
Spoon the Onion Mustard Sauce onto 8 plates. Set 2 fish cakes in the sauce on each plate, spoon the root-vegetable slaw on top and serve.