1. Cut a cross on the flat side of each chestnut. Place them in a saucepan, cover with water and bring to a boil. Simmer about 10 minutes.
2. While the chestnuts are hot, remove the outer skin and thin inner brown peel. (This is a painstaking -- and sometimes painful-- task, but the chestnuts must be hot for the inner brown skin to come off.)
3. If the chestnuts are not cooked through, simmer them in chicken stock for 8-10 minutes. (You can tell if they are cooked by breaking one open and checking to see if the inside is the same color as the outside.)
4. Set the cooked chestnuts aside. If you can't find fresh chestnuts you can use vacuum-packed whole chestnuts.
5. Trim the roots of the onions carefully without cutting across the ends. Cut a cross on the bottom of each onion.
6. Cover the onions with water in a medium saucepan and bring to a boil. Reduce heat to low and simmer until firm/tender. Drain and remove peels. Set aside.
7. Wipe the mushrooms clean with a damp paper towel and cut into thick slices.
8. Warm half the oil in a large saute pan and quickly saute the mushrooms in two batches.
9. Preheat the oven to 350°F.
10. Combine chestnuts, onions and muhrooms in a large casserole. Toss with thyme, adding chicken broth if the mixture seems dry.
11. Season with salt and pepper, bake 15 minutes until hot all the way through. Serve hot.