Although dishes containing raw eggs are not a risk for most people, they should be avoided by the elderly or infirm, infants and pregnant women.
Put egg yolks, mustard, salt, sugar and black pepper in a heatproof mixing bowl. Beat well, slowly beat in half the oil, drop by drop, until the mixture becomes thick and pale.
Gradually beat the lemon juice into the sauce mixture. Beat in the remaining oil, a tsp at a time. Beat in 1tbsp hot water.
Wash and finely chop watercress. Peel and chop onion. Stir water cress and chopped herbs into the sauce. Set the bowl over a pan of hot water.
Meanwhile, put salmon in a large frying pan. Add bay leaves. Tarragon sprigs and onion. Pour in enough water to come half-way up the sides of fish. Poach gently for 4-5 minutes on each side, or until fish is cooked. Remove fish from pan, discarding the cooking liquid, onion and herbs. Serve with the herb sauce and lemon slices.
When setting the bowl of sauce over the pan of hot water, be careful not to let the hot water touch the base of the bowl, or the sauce may curdle.
To complete the meal, serve the salmon steaks and herby sauce with salad leaves and boiled baby new potatoes.