Boil the tagliatelle in salted water almost cooked according to the instructions on the package. They should have a little more bite.
Cut the cleaned savoy cabbage and the peeled onions into fine strips. Sauté both in a large pan with a little butter. As soon as the vegetables have got color, deglaze with approx. 250 ml vegetable stock. Let it simmer gently for about 15 minutes over a low heat. When the vegetable stock has boiled off, pour on the cream. Season to taste with salt and pepper.
Then add the diced salmon seasoned with salt, pepper and lemon juice and let it steep. Season again to taste. Possibly stir in some cream or cheese. Then put the pasta in the pan, stir everything and heat evenly. Serve immediately.