Add oil, onions and thyme to a pan over low heat and cover with lid; steam for about 20 – 30 minutes or until onions reduce down.
Remove pan lid and continue to cook for approximately 90 minutes; check every 30 minutes or so, until the last 30 minutes when the onions will need stirring more often to avoid black bits or burning.
When the onions are a deep all over butterscotch color, add the tomato paste.
Use a splash of stock to deglaze the pan, then add the rest of the stock before simmering over low heat for an hour.
Make sure soup is heated through thoroughly and serve.