Place the potatoes in a large pot and cover with water by 1 inch. Add 1 tablespoon salt and bring to a boil.
Lower the heat and simmer for 15 to 25 minutes, depending on the size of the potatoes, until just tender. (I use a small bamboo skewer to test them.)
Drain in a colander, cover the colander with a clean kitchen towel, and allow the potatoes to steam for 5 to 10 minutes. Cut them in quarters or halves, depending on their size, and place them in a large bowl.
Meanwhile, whisk together the vinegar, mustard, garlic, egg yolk, 2 teaspoons salt and 1 teaspoon pepper. While whisking, slowly pour in the olive oil, making an emulsion.
Stir in the wine and capers. While the potatoes are still very warm, pour half the vinaigrette on the potatoes and toss them gently, allowing them to soak up the vinaigrette.
Stir in the spring onions, celery, red onion and lobster and add enough vinaigrette to moisten. Reserve any remaining vinaigrette. Add the zest and juice of the lemon, the tarragon, 2 teaspoons salt and 1 teaspoon pepper and toss carefully.
Cover with plastic wrap and refrigerate for at least an hour to allow the flavors to blend. Taste for seasonings and add more vinaigrette, if necessary. Serve at room temperature.