Adjust oven rack to 6 inches below heat source. Heat broiler.
Place the potatoes in a 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, then cut into quarters.
Meanwhile, in a bowl, combine the crème fraîche, 1 tablespoon lemon juice, 1 teaspoon salt, and ¼ teaspoon pepper. Stir in the potatoes and fennel.
Place the salmon on a parchment- or foil-lined baking sheet. In a bowl, combine the mustard and remaining lemon juice. Spread over the top of the salmon.
Broil the salmon until it 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.