1. Brush the bottom of a large deep frying pan with 1 tablespoon of the oil. Put the potatoes in the pan and sprinkle with 1/2 teaspoon of the salt. Top with the onion and then the salmon. Sprinkle another 1/2 teaspoon of the salt and 1/4 teaspoon of the pepper over the salmon. Add the water to the pan, cover, and bring to a boil. Reduce the heat and simmer until the salmon and potatoes are done, about 15 minutes.
2. Remove the salmon and flake. Drain the potatoes well and put in a medium bowl. Add the cream and mash, leaving the potatoes fairly chunky. Add the salmon, another 1/2 teaspoon of the pepper, the scallions, and 2 tablespoons of the dill. Form the mixture into eight cakes; they needn't be perfectly symmetrical or smooth.
3. Wipe out the frying pan. Add 1 tablespoon of the oil and heat over moderately high heat. Add half the salmon cakes to the pan and brown well on both sides, about 5 minutes in all. Drain on paper towels and repeat with another tablespoon oil and the remaining salmon cakes.
4. In a medium glass or stainless-steel bowl, whisk together the mustard, vinegar, and the remaining 1 tablespoon dill and 1/4 teaspoon each salt and pepper. Add the remaining 1/4 cup oil slowly, whisking. Add the greens, toss, and put on plates. Top each salad with two salmon cakes and a lemon wedge.
Wine Recommendation: The full flavors of this salad can take on a chardonnay that would drown out other dishes. Select a bottle from California's Napa or Sonoma Valley for a memorable treat.