Preheat the over to 190°C fan (400°F), Gas Mark 6.
Soak the wild mushrooms in 75ml (2½fl oz) just-boiled water for 15 minutes.
To make the sage butter, mash the butter with the sage and garlic and set it aside (I only chill this if I’m going to keep it for a while).
Drain the wild mushrooms, adding their soaking liquid to the chicken stock. Wash the rice in a sieve under the cold tap, until the water runs clear, to remove the excess starch.
Put the onion and both the chestnut and dried mushrooms into a 30cm (12in) sauté pan or shallow casserole (the width is very important) and sprinkle on the rice (it may not look like much, but it expands, don’t worry). Put the chicken thighs, skin side up, and the pumpkin on top. Sprinkle a little olive oil over the vegetables and chicken and season well. Bring the stock mixture to the boil, then carefully pour it around the chicken thighs.
Bake in the oven for 45 minutes, by which time the chicken will be lovely and golden and the stock will have been absorbed. Put knobs of the sage butter over the chicken thighs, allow it to melt, then serve.