Cook the tagliatelle in salted water according to the instructions on the packet.
Melt the margarine for the sauce, sauté the finely chopped onion and the pressed garlic in it and dust with flour. Let the flour brown slightly, then pour in the milk and bring to the boil, stirring vigorously. Be careful not to create any lumps. Season the sauce with mustard, herbs and spices. Add the salmon and let it simmer in the simmering sauce for about 10 minutes.
Place the finished salmon sauce over the tagliatelle on plates.