Boil the eggs. Place eggs in cold water. Bring to a boil, then cook for 9 minutes. Transfer immediately to ice water for 5 minutes. Peel and set aside.
Cook the potatoes (if using). Boil in salted water until almost tender — about 12–15 minutes. A small firm core should remain. Drain and cool completely. Dice into ½-inch (1 cm) cubes.
Dice the eggs. Chop the peeled eggs into rough ½-inch (1 cm) pieces. Do not mash them.
Chop the sprats. Roughly chop the drained sprat fillets into small pieces.
Make the dressing. In a large bowl, combine the crème fraîche, mayonnaise, and the reserved tablespoon of sprat brine. Stir until smooth.
Combine. Fold in the eggs, sprats, potatoes, dill, and chives gently. Season with white pepper and a pinch of salt — the sprats are already salty, so taste before adding more.
Chill. Cover and refrigerate for at least 30 minutes. For best results, chill overnight.
Add onion just before serving. Stir in the diced red onion immediately before serving to keep it crisp and fresh-tasting.
Serve on dark rye bread, crispbread, or as part of a smörgåsbord.
