Preheat the oven to 220 C, 200 Fan, Gas Marl 7. Line a baking tray with baking paper.
Spread the potatoes out onto the lined tray along with the 4 smashed cloves garlic, the olive oil and the thyme, Season with salt and pepper. Roast for 20-30 minutes until the potatoes are softened but not yet fully cooked through.
Add the mushrooms, sliced garlic and sherry to the potatoes and mix well to distribute evenly. Cook for a further 25-30 minutes until the mushrooms and potatoes are cooked through and have taken on some colour.
Remove from the oven and add the chestnuts and parsley, mixing well. Check the seasoning adding more if you like.
Using a potato peeler shave the manchego or parmesan over the vegetables , sprinkle with more oil if you like and serve.