Preheat oven to 375 degrees F.
Place the salmon in a parchment paper-lined baking dish. Drizzle with lemon juice and olive oil; season with salt and pepper. Bake 15 minutes, or until just cooked through. Cool salmon to room temperature, and then refrigerate at least 1 hour.
Boil the potatoes until just tender and then drain well, spread onto a plate, and cool to room temperature.
Combine the mayonnaise, sour cream, vinegar, sugar, mustard and tarragon in a medium bowl. Season the mixture with salt and pepper. Mix in the potatoes, cucumber, green onion and carrot. Cover and refrigerate until needed.
When ready to serve, divide the salad among 4 plates. Set a piece of salmon on each plate, garnish with lemon and parsley, and serve.