1. In a small bowl, soak the porcini in 300ml of boiling water for a quarter of an hour. Pass the liquid through a strainer into a jug, adding boiling water to reach 600ml. Dissolve the stock cube or powder into the liquid and chop the porcini.
2. In a shallow, non-stick pan, heat a spoonful of oil and fry the chestnut mushrooms, keeping the heat high. Once golden and shrunken, which enhances the flavour, remove them from the pan into a bowl, then wipe the pan.
3. With the pan still on the heat, add another spoonful of oil and the butter. Cook the shallots and garlic until softened. Add the porcini and risotto rice, stirring until they're coated. Pour in the wine and reduce until fully absorbed. Gradually add the stock, stirring until the rice is almost ready. Return the mushrooms to the pan, pour in the remaining stock with the spinach, incorporate the pecorino and zest, then remove from the heat, cover, and leave for 5 minutes. Serve in bowls, garnishing with more cheese if desired.