Heat the oil in a pan. Peel the onions and garlic, cut into small pieces and add to the pan. Dice the salmon and add to the pan as well. Pour a squeeze of lemon juice over it. Wash the tomatoes, cut them into small pieces and also add to the pan. When the salmon is pink, add the sour cream or crème fraiche (tastier in my opinion) to the pan. Possibly add a shot of white wine. Season the whole thing with salt and pepper.
Put the tagliatelle in the boiling water (the fresh ones only need 2 minutes) and cook. As soon as they are done, drain them through a sieve and simply add them to the pan. Mix with the sauce and serve hot.
It tastes really delicious and is ready in no time.