Preheat the oven to 200°C/Fan 180°C/ Gas 6. Line a baking tray with nonstick baking paper. Spread the chestnut mushrooms out on the lined baking tray, season to taste and drizzle with olive oil. Roast for 10 minutes or until cooked through. Set aside.
Meanwhile, put the dried mushrooms in a pan, cover with the measured water and bring to the boil over a medium heat. Remove from the heat and leave the mushrooms to soak for 5–10 minutes or until rehydrated. Strain the liquid into a jug and reserve. Roughly chop the rehydrated mushrooms and set aside.
Pour the stock into a pan and bring to a gentle simmer. Melt 100g of the butter in a wide pan over a low heat. Add the onion and sweat for about 10 minutes until soft and translucent. Then add the sage and thyme and cook for a further 5 minutes.
Add the rice and stir until all the grains are well coated in the onion mixture. Then stir in the wine, turn up the heat and cook for 5 minutes until it has evaporated.
Gradually add the hot stock, a ladleful at a time, stirring after each addition until it is almost all absorbed, before adding the next. Continue until all the stock has been used up.
Add the mushroom soaking liquid and cook, stirring, until almost all absorbed and the rice is al dente – the whole process should take about 20–25 minutes. If you are freezing a batch, cook the rice for 2–3 minutes less.
Remove from the heat and beat in the grated Parmesan and the remaining butter until melted and incorporated. Then stir through all the mushrooms and parsley, season to taste and garnish with Parmesan shavings.
To freeze, portion out the risotto into appropriate containers, adding 2 tablespoons of water to each portion. Leave to cool, then seal, label and date before freezing. To reheat, microwave on high for 3 minutes, stir, cover (but don’t seal) and microwave for three minutes. Reheat in the oven by covering the top in foil and cooking for 30-35 minutes on 180°C/Fan 160°C/ Gas 5.