Adjust oven rack to 6 inches below heat source; heat broiler.
Place the potatoes in a medium pot, cover with cold water, and bring to a boil. Add 1 teaspoon salt, reduce heat, and simmer until tender, about 15 minutes. Drain, run under cold water to cool, then cut into quarters.
Meanwhile, in a large bowl, combine the crème fraîche or sour cream, 1 tablespoon of the lemon juice, 1 teaspoon salt, and 1/4 teaspoon pepper. Stir in the potatoes and fennel; set aside.
Place the salmon on a parchment- or foil-lined baking sheet.
In a small bowl, combine the mustard and the remaining lemon juice. Spread over the top of the salmon. Broil until the salmon is the same color throughout, 6 to 8 minutes, depending on the thickness. Divide the salmon and salad among individual plates and sprinkle with the dill.
Upgrade: For a snappy, licoricey boost, reserve the feathery green fronds of the fennel and mix them with the dill before garnishing.