Preheat the oven to Gas Mark 2/170°C/Fan 150°C.
Place the salmon on a large sheet of greased tin foil. Set this in a large baking tray. Place the onion, lemon and bay leaves inside the cavity, scattering any extras around the fish, and season. Then drizzle over the white wine.
Bring the foil up and over the salmon to loosely seal (you may have to use another sheet of foil). Cook in the oven for 1 hr. Then leave to cool.
Meanwhile place the watercress in a sieve and pour over a kettleful of boiling water. Squeeze away excess water and leave to cool completely.
Place the egg yolks in a food processor with the mustard and whiz together. Then slowly start to mix in the oil, just 1 drop at a time to start with, gradually adding a little more.
Add the watercress and vinegar and whiz until smooth. Then season and add the lemon juice to taste.
To serve, carefully lift the fish onto a serving platter. Remove the head. Then carefully peel away the skin from the serving side of the fish and ease away the dark flesh running along the backbone with a teaspoon.
Arrange the lemon and cucumber around along with the mayonnaise in a bowl.
To serve, cut away chunky portions of the top side of fish. Then remove the flavourings from the cavity along with the bones. Portion out the bottom side of fish, easing it away from the skin which can be left behind.