Heat oven to 150C/fan 130C/gas 2. Put the salmon on a large sheet of oiled extra-wide foil (this will stop it from sticking later). Put the onion, bay leaves and half the dill in the body cavity, spoon over the wine, season with salt and black pepper, then loosely bring the foil round the salmon and seal well to make a parcel. Lift onto a baking tray, then bake for 2 hrs. Take from the oven, then leave to cool for about 10 mins, still in its parcel.
Meanwhile, whizz the flesh from the avocados (with a hand blender or in a food processor) with the mayo, lemon juice, zest and seasoning. Tip into a bowl. Chop the remaining dill, stir in with the cucumber, then transfer to a serving bowl. Chill for up to 3 hrs.
Unwrap the salmon, then strip away the skin and fins. Personally, I don’t take the skin from the bottom of the fish if I am serving the salmon warm, as you risk breaking the fish up as you turn it over. It is more robust when it is cold, so in this case remove the skin from both sides. You can leave the head on or take off at this stage. Carefully lift onto a platter, then garnish with watercress, cucumber, lemon wedges and dill, if using.
To serve, remove the fish in chunky fillets. When all the fish has gone from the top fillet, remove the onion and herb flavouring, pull away the bones, then remove portions of the bottom fillet, leaving the skin behind. Eat with the avocado mayo, warm buttery new potatoes, and salads or green beans.