Step 1 Place the potatoes in a saucepan, add cold water to cover and bring to a boil over medium-high heat. Add 2 teaspoons of salt, and reduce the heat so the water just simmers. Cook the potatoes until tender when pierced with a fork, about 12 to 15 minutes. Drain the potatoes and run them under cold water until cool enough to handle.
Step 2 While the potatoes are cooking, whisk together the shallot, mustard, sugar, the remaining 1/2 teaspoon of salt, the lemon juice and olive oil in a small bowl. Add pepper to taste.
Step 3Cut the potatoes into thin slices and place in a large salad bowl. Use a fork to flake the salmon into bite-size pieces. Add to the salad bowl along with the dill and avocado. Cut off and discard the tough stems of the watercress. Coarsely chop the rest and add to the salad bowl. Gently toss the salad with the dressing. Divide the salad among 4 plates. Serve immediately.