1. Check the fillets carefully for bones and remove any you find with tweezers.
2. Place the fish into a dish; rub with salt, allowing 15 g (1 tbsp.) per kilogram of salmon; season with pepper;
3. brush with oil and lemon juice; sprinkle with chopped dill; 
4. cover with plastic wrap and marinate for 12 hours in the refrigerator.

Assembly and refrigeration

1. Dissolve the gelatin in 125 ml (1/2 cup) water;
2. purée the smoked salmon in a blender with the gelatin;
3. whip the cream to stiff peaks; fold into the salmon mixture;
4. slice the marinated salmon into 12 wide thin slices; place on a work surface; 
5. divide the smoked salmon mousse among the salmon slices, preferably with a piping bag, or else use a spoon to lay out a long cylinder of mousse the length of the slice;
6. roll up gently; place on a platter; cover with plastic wrap and refrigerate for at least 3 hours;
7. just before serving, peel and slice the avocados; arrange the slices from half an avocado in a fan shape on each plate; top with two salmon rolls and serve cold.