Place the gnocchi in boiling salted water for 1 minute, drain and leave to drain. Peel and finely chop the onion. Heat the fat in a saucepan. Sauté onion for about 1 minute. Add flour, deglaze with milk while stirring, bring to the boil, stir in stock and simmer for about 3 minutes.
Season with salt and pepper. Cut salmon into strips. Add gnocchi and salmon to the sauce and heat. Season with lemon juice. Clean and wash the rocket. Arrange on the gnocchi.