How to Make It
Bring a large pot of salted water to a boil. Add the potatoes and cook for 8 minutes. Stir in the peas and continue cooking until the potatoes are tender, about 2 minutes longer. Drain thoroughly.
Meanwhile, fill a large, deep frying pan with 3 inches of salted water. Bring to a simmer. Add the salmon and simmer until the fish is just cooked through, about 10 minutes. Transfer the salmon to paper towels to drain. Cut or flake the salmon into bite-size pieces.
Chop enough of the fennel fronds to make 1 tablespoon. In a medium glass or stainless-steel bowl, whisk together the sour cream, mayonnaise, chives, oil, lemon juice, chopped fennel fronds, salt, and pepper.
In a large bowl, toss the watercress with 1/4 cup of the dressing and put the greens on plates. In the same bowl, gently toss the potatoes and peas, the salmon, and the sliced fennel with the remaining dressing. Top the watercress with the potato-and-salmon mixture.