Set broiler on high. Place the salmon on a foil-lined broiler pan and season with 1/4 teaspoon of the salt. Broil until the salmon is the same color throughout and flakes easily, 7 to 10 minutes, depending on thickness.

Meanwhile, in a medium saucepan, over medium-high heat, heat the oil and 1 tablespoon of the butter until it melts. Add the shallot and cook until softened, about 1 minute. Add the wine and cook until reduced by half, about 3 minutes. Reduce heat to low and whisk in the mustard, dill, pepper, and the remaining salt. Remove from heat.

Cut the remaining butter into pieces, add to the sauce, and whisk until incorporated. Place the salmon on individual plates, spoon the sauce over the top, and sprinkle with additional dill. Serve with the cucumber (if desired).