Step 1Combine the salad: In a large bowl, coarsely shred the fish using your fingers, making sure to remove any pin bones. Stir in the onion, cucumber, chopped walnuts, green onion and basil.
Step 2In a medium bowl, whisk together the vinegar, mustard, olive oil and honey. Whisk in the dill. Taste and season as desired with salt. To the salad mixture, add 5 tablespoons plus 1 teaspoon dressing and mix well. Taste and adjust the seasoning if desired.
Step 3Line a baking sheet with a piece of parchment paper. Divide the salad into 8 portions, and form each portion into a ball. (The recipe can be prepared up to this point several hours in advance. Cover and chill.)
Step 4To serve, place a ball on a serving plate, and drizzle 1 to 1 1/2 tablespoons of the remaining dressing around the ball. Garnish the plate with a few slices of apple arranged in a fan-like shape and 3 walnut halves, if using. Garnish the top of each ball with chives. Repeat with the remaining salad balls and serve immediately.