Set broiler on high. Place the salmon on a foil-lined broiler pan, season with ¼ teaspoon salt, and broil until it is the same color throughout and flakes easily, 7 to 10 minutes, depending on thickness.
Meanwhile, in a saucepan, over medium-high heat, heat the oil and 1 tablespoon butter until it melts. Add the shallot and cook until soft, 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, ¼ teaspoon salt, and ⅛ teaspoon pepper. Remove from heat.
Cut the remaining 2 tablespoons butter into pieces, add to the sauce, and whisk until incorporated.
Place the salmon on individual plates, spoon the sauce over top, and sprinkle with additional dill. Serve with the cucumber slices (if desired).