To make the sauce, heat a tablespoon of the butter in a saucepan and sweat the shallots for 3–4 minutes, or until softened. Pour in the Sauternes and cook until reduced by half. Pour in the chicken stock and cook until reduced by half again. Pour in the fish stock and reduce by half again. Pour in the cream and bring the mixture to the boil. Whisk in the rest of the butter and finally stir in the caviar. Season with salt and pepper and keep warm.
Warm the oyster juice in saucepan with a squeeze of lemon juice. Add the oysters and remove from the heat. Set aside.
Preheat the oven to 200C/180C Fan/Gas 6. Bring a saucepan of water to the boil and add a squeeze of lemon juice. Cook the salsify for 6–8 minutes, or until tender. Drain and pat dry with kitchen paper. Transfer to a roasting tin and dot over half the butter. Roast for 2–3 minutes, or until golden. Set aside and keep the oven on.
Season the flour with salt and pepper. Dust the fish in the flour and cook in a pan for 6 minutes, until just cooked through. Add the remaining butter and baste the fish well. Squeeze over the remaining lemon juice.
To serve, place some spinach on a serving plate, top with the fish and the salsify. Stir the oyster mixture into the caviar sauce and spoon over the fish.