Pat capers dry and fry briefly until crispy in a small pan in hot oil. Drain on paper towels.
Cook the eggs until hardboiled, about 10 minutes. Rinse in cold water, peel and cut in half lengthwise. Pat herring dry and cut into 8 pieces.
For the dressing, peel the onion, cut into quarters and cut into strips. Cut the cucumber into small pieces. Stir mustard with the honey, lemon juice and oil and season with salt and pepper. Mix in onion and cucumber and season to taste.
Cut the bread into 4 wide strips and brush with butter. Top with sliced eggs and herring. Drizzle with dressing and serve sprinkled with chives and capers.