Combine salmon, onion, eggs and egg white, mustard, 2 tablespoons fresh dill (or 2 teaspoons dried), pepper and salt in a large bowl. Add potatoes and stir to combine.
Preheat oven to 200°F.
Heat 1 tablespoon 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. Cover and cook until browned on the bottom, 3 to 5 minutes. Gently turn over and cook, covered, until crispy on the other side, 3 to 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 oil and the remaining salmon mixture.
Combine sour cream, capers, lemon juice and the remaining dill in a small bowl. Serve the salmon cakes with the dill sauce.