Peel the potatoes and cut into cubes. Bring to the boil in a pan of salted water. Cook for 10 minutes until cooked.
In the meantime, wash the asparagus and remove the stalk end. Chop the watercress (set some aside for garnish) and lamb’s lettuce.
Season the fish with salt and pepper on both sides. Fry in a pan with a little butter and oil on both sides, starting with the skin side, for 3 to 4 minutes, depending on the thickness of the salmon.
Heat the onion and garlic in another pan with olive oil and fry until they are translucent. Add the asparagus and cook for 2 to 3 minutes. Cover and cook further.
Remove the asparagus from the pan but leave the onion in the pan. Keep the asparagus warm with the salmon.
Add the mustard and vegetable cream to the pan. Reduce slowly and season with salt and pepper.
In the meantime, drain the potatoes and mash with a potato masher, adding a knob of butter and oil until smooth. Mix in the chopped watercress and lettuce and stir well. Season with nutmeg, salt and pepper.
Serve the fish with the asparagus in mustard sauce, herb mashed potatoes and extra watercress.