Thinly slice up the onions. Add to a large pan over a low heat with the oil. Cook, stirring often, until they are starting to go golden brown. This could take 20 minutes or so. Add the thyme and continue to cook until a deep golden brown, this could take another 20 minutes or more. Add the crumbled stock, a few dashes of Worcestershire sauce, stir and cook for another few minutes. Taste for seasoning.