Fry the onion in the oil for 5 mins. Add the rice and garlic, then cook for 2 mins, stirring continuously. Pour in a third of the stock and set the timer to 20 mins. Simmer, stirring occasionally, until the stock has been absorbed, then add half the rest of the stock and carry on cooking, stirring a bit more frequently, until that has been absorbed.
Pour in the last of the stock, stir, then simmer until cooked and creamy. Take from the heat and add the chopped salmon, mascarpone, parsley and lemon zest. Grind in some black pepper, but don’t add salt as the salmon will be salty enough.
Leave for 5 mins to settle, then taste and add a little lemon juice if you like. Serve topped with reserved salmon (roughly torn) and some rocket.