Bring a large pan of salted water to a boil and simmer the calf’s tongue for 2 hours, until soft and cooked through. When cool enough to handle, peel the tongue and place it back in the cooking liquid to cool completely.
Heat 1 tablespoon of the oil and the butter in a saucepan over a medium heat and cook the shallots and garlic until softened. Add the white wine vinegar and evaporate. Add the white wine and reduce by three quarters. Add the stock and reduce by half. Stir in the cream, mustards and most of the chives and season with salt and pepper. Keep warm.
Put the flour in a bowl and season with salt and pepper. Slice the tongue and coat in the seasoned flour.
Heat the remaining olive oil in a frying pan and fry the tongue slices for 5 minutes on each side or until golden-brown and crisp. Drain on kitchen paper.
Fry the turbot pieces in the same pan for 5 minutes then add the butter and heat until foaming. Add a squeeze of lemon and the thyme and baste the turbot.
To serve, layer the turbot and tongue between two plates and pour over the mustard sauce.