In a large sauté pan heat the butter and the oil to a medium high heat. Add the onions and sauté until translucent and soft, stirring often, about 10 minutes. Turn heat down to medium and continue to sauté slowly stirring for another 10 minutes. Add the thyme and the parsley leaves and continue to sauté, they should be turning brown now and caramelizing. Make sure you scrape the bottom the pan and stir often. Add the Madeira, port or sherry and a little more butter. Sauté a few more minutes until well caramelized and brown.