Peel and finely chop the onion. Heat the fat in a saucepan and fry the onion until transparent. Add cream and stock and bring to the boil. Add lemon zest, sprinkle in sauce thickener while stirring. Bring to the boil again briefly, season to taste with salt, pepper and lemon juice. Cut salmon into pieces and add to the hot sauce.
Cook tagliatelle in plenty of boiling salted water for about 6 minutes. Wash the dill, dab dry and chop finely, except for 2-3 little flags for garnishing. Pour noodles into a sieve and drain. Put them on a plate and pour cream sauce over them. Sprinkle with dill and serve garnished with a slice of lemon and dill flags