In a frying pan, gently fry the onion in the oil until softened. Tip in the risotto rice and cook for 2 mins, stirring continuously. Add a third of the stock, and simmer, gently stirring until the stock has been absorbed. Add half the remaining stock and carry on cooking, stirring a bit more, until that has been absorbed too.
Tip in the final lot of stock and cook until creamy and the rice is just tender. Stir in the trout, mascarpone, lemon zest, most of the chives and some seasoning – it won’t need too much salt. Cover and leave for 5 mins to rest. Finally season with a squeeze of lemon juice, divide between two bowls and top with the remaining chives. Great with a green salad on the side.