Add the onions to the same pan and, stirring constantly with a wooden spoon, cook them until lightly browned, 5 – 7 minutes. Add the wine, stock, tomato sauce, and thyme and bring the mixture to a boil. Return the oxtails to the pot, submerging them in the liquid, and return the pot to a boil. Cover the casserole and cook in the oven for 1 – 1 1/2 hours, or until the meat is falling off the bone.