MELT the butter in a medium saucepan over medium heat.
ADD the onions, thyme, bay leaf and oregano, and sauté until tender, about 5 minutes. Sprinkle the onions with the sugar and continue to sauté until well combined.
REDUCE heat to low and continue to cook the onions, covered and stirring regularly, for 45 minutes or until completely caramelized.
DEGLAZE the onions with the sherry and reduce for 5 to 7 minutes. Add the white wine and reduce for an additional 10 minutes.
ADD the chicken stock and demi-glace and simmer over low heat for 20 minutes or until reduced slightly. Taste and adjust seasoning as necessary.
FILL a serving dish ¾ the way up to the rim with boiling onion soup, top with ½ cup of the croutons in each.
COVER the entire top of the soup and dish with the sliced cheese and spread ½ cup of the mixed grated cheeses, carefully.
BROIL in the oven until the cheese is dark golden brown.