Cook the pasta in boiling salted water for ten minutes. Peel and finely chop the onion. Wash the mushrooms, dab dry and halve or quarter them depending on size. Heat the oil in a pan. First fry the mushrooms in the oil, turning them well.
Add onions and fry until golden brown. Season with salt and pepper. Drain the pasta on a sieve. Return to the pot, melt truffle butter on top. Mix crème fraîche and lemon juice in the small cup, season with salt and pepper and stir into the mushrooms.
Pour over the ribbon noodles, mix loosely. Wash the parsley, dab dry and remove the leaves. Arrange noodles on a preheated plate. Cut salmon into wide strips and place on top.
Garnish with parsley leaves and serve sprinkled with coloured pepper.