Preparation

Make the quenelles

• In a medium bowl, mix all the ingredients with 1 tsp. salt and 1/2 tsp. pepper. Transfer to a food processor and pulse 6 to 8 times so the salmon is coarsely chopped. Don’t overchop or the texture will be spongy.
• Using a 2-oz. scoop (or large spoon) and damp hands, gently form the salmon into 16 ovular shapes; each should weigh about 2 oz. Set on a large baking sheet lined with parchment paper, cover with plastic wrap, and refrigerate for 1 hour to set.

Make the broth and poach the quenelles

• Put the water, onion, carrots, celery, parsley, and dill in a large 8-qt. pot, and bring to a boil over medium-high heat. Reduce the heat to medium-low and simmer for 10 minutes. Gently drop in the salmon quenelles, and cook, flipping after a couple of minutes, until firm but still a little translucent in the center when you make a nick into a thicker piece, about 5 minutes. Using a slotted spoon, transfer to large plate. Gently pat dry with paper towels. Cool to room temperature. Cover and refrigerate for at least 2 hours and up to 2 days before serving.

Make the vinaigrette

• Puree the beets, vinegar, horseradish, honey, and mustard in a blender. With the blender on, drizzle in the oil in a thin steady stream until smooth and emulsified. Season to taste with salt and pepper (about 1/2 tsp. each). Cover and refrigerate for up to 2 days before serving.
• To serve, in a large bowl, toss the arugula and herbs with enough vinaigrette to lightly coat. Set 2 of the salmon quenelles on each of 8 salad plates. Top with a mound of the herb salad and drizzle with the remaining vinaigrette.