1.Spray medium frying pan with cooking oil. Cook half the egg over medium heat, swirling pan to make thin omelette. Remove from pan. Cool on baking-paper-covered wire rack. Repeat with remaining egg.
2.Combine mayonnaise, dill and juice in small bowl.
3.Spread each omelette with half the flavoured mayonnaise, top with watercress, salmon and cucumber. Roll omelettes to enclose filling. Slice in half to serve, if you like.