Cook the pasta according to the instructions on the packet.
Heat the oil in a frying pan, add the onion and fry until soft and translucent but not coloured. Take the pan off the heat and add the smoked trout, lemon rind, crème fraîche and chopped dill.
Drain the pasta, reserving 2 tablespoons of the cooking water. Stir the pasta and water into the sauce. Season to taste with salt and pepper and serve immediately.