First, preheat the oven to Gas mark 6 or 200c. Wash the leek, trim the ends off and cut in half. Cut each half, length ways. Steam until tender and then place under cold water to keep the nice bright green colour.
Put half the lemon zest and marjoram in each trout and season. Wrap the trouts in the leek and place in a buttered oven-proof dish, cover in foil and bake for 10-12 minutes or until cooked through.
In the meantime, make the sauce. Melt the butter and add the watercress. Cook for a couple of minutes until it has wilted and some of the liquid released has evaporated. Add the cream, capers and seasoning and serve hot over the trout. Serve with some freshly boiled, buttered new potatoes.
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