Line a 23 x 30 cm (9 x 12 inch) roasting tin with nonstick baking paper.
Melt the butter in a saucepan, stir in the flour and cook for 1 minute. Gradually mix in the milk and bring to the boil, stirring until thickened and smooth. Remove from the heat and stir in the egg yolks, watercress, lime rind and salt and pepper. Leave to cool for 15 minutes.
Whisk the egg whites into stiff peaks. Fold a large spoonful into the cooled sauce to loosen the mixture, then fold in the remaining egg whites. Spoon the mixture into the prepared tin and ease into the corners.
Bake the roulade in a preheated oven, 180°C (350°F), Gas Mark 4, for 15–20 minutes until it is well risen, golden brown and the top feels spongy. Cover with a clean tea towel and leave to cool for at least 1 hour.
Meanwhile, steam the salmon for 8–10 minutes until it is cooked. Leave to cool, before skinning, flaking and discarding any bones. Beat the crème fraîche with the lime juice and plenty of salt and pepper.
Place a large piece of baking paper on the work surface so that a short edge is nearest to you and sprinkle with the Parmesan. Turn the cooled roulade out on to the paper, remove the tin and peel away the lining paper.
Spread the roulade with the crème fraîche mixture, then the salmon. Roll up the roulade, starting from the shortest side nearest to you, using the paper to help. Place in the refrigerator for a least 30 minutes, then cut into thick slices and serve garnished with a few watercress leaves and lime wedges, if liked.