Cut the onions in half horizontally making sure the root is at the bottom. Trim the root and top to make the onions stand up.
Using a sharp knife remove the centre parts from the onions leaving 3 good layers to make an onion "cup".
Place the onions in a casserole dish and brush with olive oil.
In a pan heat the tbsp olive oil and the butter and fry the celery, garlic and the cuttings from the onion centres. Cover the pan and cook for 15 minutes so soft but not browned.
Add the chestnuts to the onion mixture breaking them up slightly. Add thyme and polenta and stir together.
Place the onion mixture into the cups, packing them well. Bake for 30 minutes until the cups are tender (this may vary depending on the size of onion used) If the stuffing seems to be drying cover with foil.
While the onions are cooking make the gravy. Place the mushrooms in a saucepan with the stock and bring to the boil. Remove from heat and leave to soak for 15 minutes. After 15 minutes remove and chop the mushrooms.
Fry the onion in the olive oil for 10 minutes until golden. Add the mushrooms and garlic and fry for a further 2 minutes.
Add flour and mix well. Add the soy and the stock and cook until thickened.
You can either puree the gravy or leave chunky.
Simmer the gravy for 10 minutes and serve poured over the baked onions.