Sauté the onion in the butter until soft and allow to cool. Place the onion, chicken meat and egg white in a food processor and purée until smooth. Spoon the mixture into a mixing bowl. Add the breadcrumbs, herbs and créme fraîche and season with salt and pepper to taste. Refrigerate until needed.
Chicken Cut a pocket in the chicken breasts by making a slit in one side of each. Season with salt and pepper and spoon the stuffing into the pockets. Firmly wrap each fillet in clingfilm, twisting the ends to secure. Each fillet will resemble a sausage. Poach the chicken parcels in the chicken stock for 20 minutes and leave them to cool in the stock. Remove from the stock and refrigerate until needed.
Sauce Drain the tuna, reserving the oil. Add enough sunflower oil to the tuna oil to make up 130 ml. Make a mayonnaise by blitzing the egg, lemon juice, brown sugar, salt, the 25 ml oil and garlic in a food processor until well blended.
While the machine is running add the 130 ml oil in a thin stream. It?s important to add the oil very slowly or the mixture will separate.
Spoon the mayonnaise into a container and set aside. Blitz the drained tuna in the same food processor until very fine and add to the mayonnaise. Dilute with the white wine if necessary. Add the capers and season with salt if necessary.
Unwrap the chicken parcels and slice them. Arrange the slices on a serving platter and spoon over the tonnato sauce.