In a large and wide pot, melt butter and oil together and add all of your onions.
Cover pot and cook onions on medium heat for 15-20 minutes until they begin to break down and caramelize.
Then take the cover off and add sugar and salt. Stir in and continue to cook the onions, stirring them every few minutes until the onions continue to break down and caramelize.
Once onions are a deep brown color, add flour and fresh thyme and stir it in. The onions will get thicker and make sure you cook the flour off.
Add the white wine and allow to cook until wine reduces, about 2-3 minutes and then add your beef stock and mix well. Allow to cook on a gentle simmer for an additional 20 minutes. Season with salt and pepper and adjust seasoning as needed.
When ready, ladle soup into oven safe soup pots and top with toasted bread and shredded cheese.
Place soup bowls on a baking sheet and bake in 400 degree F oven for about 10 minutes or until the cheese melts.