Heat a large heavy saucepan. Add the oil, butter, and chopped onions. When onions turn slightly brown, deglaze with port wine. Add thyme, chestnuts, chicken stock, and a little salt and pepper. Cover and cook until chestnuts are tender and the chestnuts have absorbed most of the liquid.
Serve as whole braised chestnuts or pass through a ricer and make a wonderful chestnut puree.