Tip the salmon into a bowl and mash with a fork. Add the soft cheese and cucumber and mix together, seasoning with a little black pepper.

Lay the tortillas on a work surface and spread the salmon mixture over them. Top with shredded lettuce and watercress, sprinkle with lemon juice (if using), then roll up. Slice in half and serve with extra salad, or wrap in cling film and keep chilled to eat later.