Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until cooked through but firm to the bite, about 12 minutes. Drain and return to the pot.
Melt butter in a saucepan over medium heat. Cook and stir leek in butter until slightly softened, 6 to 7 minutes; season with salt. Pour white wine and lemon juice over the leeks, and bring to a boil while scraping any browned bits of food from the bottom of the pan with a wooden spoon. Simmer until most of the liquid evaporates, about 5 minutes.
Stir creme fraiche, mustard, and 1 pinch cayenne pepper into the leek mixture. Reduce heat to low and let cook until warmed, about 5 minutes. Add salmon to sauce; cook and stir until salmon starts to flake and is no longer pink in the center, 2 to 3 minutes. Remove pan from heat and stir in cilantro. Pour sauce over spaghetti; toss to coat. Garnish with a pinch of cayenne pepper.