Boil vinegar, wine, peppercorns, shallots, and tarragon in a saucepan until reduced to 15 milliliters.
Whisk in water, tomato purée, and egg yolks over low heat for 4 minutes until frothy.
Slowly whisk in melted butter until the sauce thickens.
Season to taste, strain through a sieve, keep warm, and stir in parsley before serving.
Preheat the grill to high and grease a tray with olive oil.
Season sole fillets and grill skin-side up for 4 minutes until opaque and crisp.
Let the fish rest for 2 minutes.
Whisk Dijon mustard, vinegar, and rapeseed oil, then toss with watercress.
Serve two fillets per plate with watercress and top with Choron sauce.