Wash the Gravlax and dry well. Cut into tiny 3mm dice and mix with the mayonnaise; add mustard and lemon juice. Season well with freshly ground pepper. Cover and leave in the fridge for 1 hour.
Meanwhile make the cucumber salad. Combine the sliced cucumber and onion in a large bowl. Mix sugar, salt and vinegar together and pour over the cucumber and onion. Place in a tightly covered container in the refrigerator and leave for at least 1 hour.
To serve the tartare, put a 6.5cm cutter on to each white plate, fill almost full with salmon mixture and smooth a little sour cream over the top with a palette knife. Lift off the cutter and put a ring of cucumber salad around the salmon tartare. Garnish the top with tiny sprigs of dill and perhaps some dill or chive flowers.
Serve with crusty white bread or hot toast.
© 2001 Darina Allen. All rights reserved.