Cook the spaghetti in salted water according to packet instructions until al dente then drain. Place in a warm bowl.
Melt butter over low heat, gradually add flour and stir until smooth. Add the milk while stirring constantly. Add the salmon and simmer for 5 minutes. Season the sauce with salt and pepper to taste then refine with a little cream.
Pour the sauce over the pasta and mix everything well. Sprinkle with Parmesan cheese and chopped dill then serve immediately.