Cook the pasta according to the instructions on the packet.
Thinly slice the mushrooms, finely dice the onion and cut the smoked salmon into strips. Heat the oil and butter in a saucepan. Add the mushrooms and onions and sauté for about 2 minutes while turning. Dust with the flour and sauté briefly. Deglaze with vegetable stock and cream and bring to the boil briefly. Season the sauce with salt, pepper and lemon juice to taste. Carefully fold in the salmon strips.
Arrange the ribbon noodles and sauce on plates. Garnish with dill flags.