For the Choron sauce, place the white wine vinegar, white wine, black peppercorns, shallots and tarragon into a non-reactive saucepan and bring to the boil. Reduce the heat and simmer until the liquid in the pan has reduced to about 1 tablespoon.
Add the water and tomato purée to the pan and whisk to combine. Beat in the egg yolks, whisking for 3-4 minutes over a low heat, or until frothy. Gradually pour in the butter in a thin, steady stream, whisking continuously until all of the butter has been incorporated and the sauce is thickened.
Season, to taste, with salt and freshly ground black pepper, then strain the sauce through a fine sieve. Discard the solids left behind in the sieve. Keep warm until ready to serve. Stir the parsley into the sauce just before serving.
Preheat the grill to high. Lightly grease a roasting tray with olive oil.
For the lemon sole, place the sole fillets skin-side up onto the prepared roasting tray and season with salt and freshly ground black pepper. Grill for 3-4 minutes, or until the skin is crisp and golden-brown and the fish is cooked through. (When the fish is cooked, the flesh should be opaque all the way through and flake easily with a fork.) Remove the fish from the grill and set aside to rest for 1-2 minutes.
For the watercress, whisk the Dijon mustard, white wine vinegar and rapeseed oil together in a small bowl until emulsified. Season, to taste, with salt and freshly ground black pepper. Place the watercress in a bowl, pour over the dressing and toss well to coat.
To serve, place 2 fillets on each serving plate. Arrange the watercress alongside, then spoon the Choron sauce over the fish.