For the duck eggs, bring a pan of water to the boil. Using a slotted spoon, gently lower the duck eggs into the water and simmer gently for 7-8 minutes to soft-boil the eggs.
Using a slotted spoon, remove the eggs from the water and refresh in a bowl of cold water. Carefully peel off the shells, then slice each egg in half and scoop out the runny yolks into a large bowl.
Finely chop the egg whites and add to the bowl with the egg yolks. Mix well until the whites are evenly coated in the yolk. Set aside.
For the salmon, pour the olive oil into a heavy-based pan and place over a very low heat, preferably with a heat diffuser underneath the pan. Heat the oil slowly until it registers 55C/130F on a cooking thermometer.
Gently lower the salmon into the oil and cook, keeping the temperature consistent by lowering or increasing the heat as necessary, for 8-10 minutes, or until the fish is cooked through. Remove the fish with a slotted spoon and pat dry with kitchen paper.
For the watercress, bring a pan of salted water to the boil, add the watercress and blanch for 20 seconds, or until the leaves are just wilted but still bright green. Drain, then transfer to a food processor and blend to a fine purée. Pour into a warm bowl.
Place the chopped duck eggs into a pan with the butter, cream, salt and freshly ground black pepper and gently warm through over a medium heat.
To serve, spoon the watercress purée into the base of four serving bowls and cover with the chopped duck eggs. Top with a portion of salmon fillet, garnish with the chervil sprigs and serve.