Place a frying pan over a high heat. Add the butter and, when melted and foaming, add the onions. Fry for 2-3 minutes, until softened, then add the thyme and wild mushrooms. Sauté for another 2-3 minutes, until tender.
Add the cobnuts and chestnuts and fry for 1-2 minutes until heated through, then season, to taste, with salt and freshly ground black pepper.
Serve a spoonful with a plateful of roast chicken.