For the Salt Cured Trout, fillet, skin and pin bone the trout and set aside. Mix the salt, sugar and zests in a medium bowl and spread half over the base of a baking dish lined with baking paper. Lay the fillets on top of the salt and cover with the remaining cure mixture, ensuring all fish is well covered. Drizzle with the gin and place in the fridge to cure for 45 minutes.
Once cured, remove the fish from the fridge and wash under running water to remove all of the cure mix. Thinly slice fillets and dress with olive oil and extra lemon zest. Set aside, covered, in fridge until needed.
For the Watercress Velouté, heat a medium sized frypan over low heat. Add the oil and onions and sauté until softened. Add the chicken stock and cook to reduce by a third. Add the milk and bring to a simmer. Once simmering, take the mixture off the heat and set aside.
Place the watercress in a blender and about ½ cup of the stock and onion mix. Blitz, adding more liquid as necessary, to make a thin, creamy purée. Continue blitzing until completely smooth and bright green. Season to taste and set aside in a small bowl, keeping warm, until needed.
To serve, divide slices of Salt Cured Trout evenly between the serving plates. Place 3 4 spoonsful of the Watercress Velouté onto the plates next to trout. Using a stick blender, blitz the surface of the remaining Watercress Velouté to make a bubbly foam. Spoon the foam off the surface of the velouté and place a spoonful on the trout on each plate. Serve immediately.