Peel the cucumber, cut in half and scrape out the core with a spoon. Cut into 1.5 cm wide half rings.
Finely dice the onion. Heat some oil in a pan and sauté the onion in it. Add the cucumber and fry for 2 minutes. Dust over 2 teaspoons of flour and sauté briefly. Deglaze with whipped cream and vegetable stock, add the mustard and simmer over a medium heat for 8 minutes.
Cut the salmon fillet into cubes. Add to the cucumber and stew for another 5 minutes. Season with salt, pepper and a little lemon juice. Fold in the finely chopped parsley.