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Combine the salmon, onion, eggs, egg white, mustard, 2 tablespoons of fresh dill (or 2 teaspoons of dried dill), pepper, and salt in a large bowl. Add the potatoes, and stir to combine.
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Preheat the oven to 200�F.
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Heat 1 tablespoon of oil in a large nonstick skillet over medium heat until shimmering. Fill a 1-cup measure two-thirds full with the salmon mixture and firmly pack it down. Unmold into the pan and pat to form a 3-inch cake. Repeat, making 3 more cakes.
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Cover and cook until browned on the bottom, 3-5 minutes. Gently turn over and cook, covered, until crispy on the other side, 3-5 minutes more. Transfer the cakes to a baking dish; keep warm in the oven. Wipe out the skillet and cook 4 more cakes with the remaining 1 tablespoon of oil and the remaining salmon mixture.
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Combine the sour cream, capers, lemon juice, and remaining dill in a small bowl. Serve the salmon cakes with the dill sauce.