Preheat oven to 200°F / 100°C.
In a large bowl, lightly beat the eggs with a fork. Add chopped onion, mustard, 2 tblsp fresh dill (or 1 teaspoons dried), breadcrumbs, pepper and salt, and stir. Add flaked smoked salmon, then shred the potatoes into it. Stir everything to combine.
Heat 1 tblsp oil in a large nonstick frying pan over medium heat until hot. Fill a 1-cup measure (or a small ramequin) two-thirds full with the salmon mixture and firmly pack it down. Unmold into the pan and pat to form a little patties. 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 and keep them warm in the oven, covered in tinfoil to avoid drying out. Wipe out the frying pan and, with the remaining 1 tblsp of oil, fry off 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.