Place the pollock fillets onto a baking tray and sprinkle over the salt. Cover with cling film and marinade in the fridge overnight. Rinse the salt off the pollock and pat dry.
Place the pollock fillet onto a large piece of cling film, then roll into a cylinder lengthways, wrap in cling film and chill in the fridge for an hour.
Preheat the oven to 200C/400F/Gas 6.
Cook the asparagus in boiling salted water for 2-3 minutes, or until tender. Drain and refresh in iced water.
Remove the pollock from the fridge, remove the cling film and cut into four pieces. Season lightly with salt and freshly ground black pepper.
Heat an ovenproof non-stick frying pan until hot, add one tablespoon of the olive oil and half of the butter. When the butter is foaming, add the pollock pieces and fry for 1-2 minutes, or until golden-brown. Turn the pollack over and roast in the oven for 4-5 minutes, or until the fish is cooked through.
Meanwhile, heat the remaining oil and butter in a frying pan and stir-fry the wild garlic for 2-3 minutes, or until wilted. Season with salt and freshly ground black pepper. Stir in the asparagus.
For the white wine sauce, heat 20g/¾oz of the butter in a saucepan and fry the shallots with a pinch of salt for 2-3 minutes, or until softened. Add the mushrooms and fry for further 1-2 minutes. Add the fish stock, bring to a simmer and cook until the volume of the liquid has reduced by half.
In a separate pan, heat the wine until the volume of the liquid has reduced by a third then add to the reduced stock mixture. Add the cream, bring to the boil and continue to cook until the sauce has thickened enough to coat the back of a spoon. Strain the sauce through a sieve, whisk in the remaining 20g/¾oz of the butter and season to taste with salt and freshly ground black pepper.
Heat a knob of butter in a frying pan and fry the morels for 4-5 minutes or until cooked through, then stir into the sauce.
To serve, squeeze a little lemon juice over the pollock and place in the middle of serving bowls. Spoon the wild garlic and asparagus around, then spoon over the white wine sauce. Drizzle with tarragon oil.
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James Martin presents a compilation of favourite Saturday Kitchen moments.