Sara Moulton's Smoked Salmon on Crispy Potato Pancakes

Potato pancakes: Peel and coarsely grate the potato. Working in batches, heat 2 teaspoons of the butter with 2 teaspoons of the vegetable oil in a large nonstick skillet over medium-high heat until very hot. Sprinkle 2 tablespoons of the grated potatoes into the pan, pressing into a 3-inch diameter with a spatula to create lacy, flat rounds of potato. Don’t be concerned if you can see the bottom of the pan through gaps in the potatoes. Cook until well browned, pressing down firmly, 5 to 7 minutes per side. Season with salt while hot. (The cakes can be made several hours in advance and crisped in a hot oven before serving.) Combine the crème fraîche, lemon juice, and horseradish in a small bowl and season with salt and pepper. Pickled onions: Combine the onions, vinegar, garlic, sugar, pickling spice, and salt in a small saucepan. Bring to a boil over high heat. Reduce the heat to medium and simmer for 2 minutes. Remove from heat and cool to room temperature before serving. Chop fine. Arrange 3 pancakes per serving on small plates. Top the pancakes with equal amounts of smoked salmon and a spoonful of horseradish cream. Top with a few pickled onions and serve.