Preheat the oven to gas mark 5, 375°F (190°C).

Wash the fish fillets and pat dry with kitchen paper. Put them in a lightly oiled shallow baking dish and sprinkle with a shake of freshly ground black pepper and half the lemon juice.

Cover with foil and bake in the oven for about 20 minutes, according to the thickness of the fillets.

Meanwhile, make the sauce. Gently sweat the onion or shallot in the butter in a covered pan over a very low heat.

When it is pale and soft, add the peas and 1 or 2 teaspoons water and cook gently for a further 2 to 3 minutes.

Tip the peas and onion into a food processor or blender, add the mint and yoghurt and process for 30 seconds until smooth.

Add the remaining lemon juice and black pepper to taste.

Transfer the fillets to warmed plates, pour a little of the sauce over each one and garnish with mint.