Melt the butter in a large frying pan, add the fish and sprinkle it with salt. Cook gently for 5 minutes, then turn the fish and cook for a further 4-5 minutes or until the thickest flesh flakes easily when tested with a fork.

With a fish slice, transfer the fish to a large serving dish and keep warm.

Add the mushrooms and lemon juice to the pan and cook for about 5 minutes until the mushrooms are tender. Remove them from the pan with a slotted spoon and reserve.

Add the spinach to the juices in the pan and cook for 2-3 minutes or until tender, stirring occasionally. Stir in the mushrooms and cook for a further few minutes to heat them through.

Spoon the vegetables round the fish and pour the juices remaining in the pan over it. Garnish with the lemon wedges before serving.