Slice the onions and leeks – feel free to use any combination of onions you would like.

Now, add all of your onions to a large dutch oven or heavy bottom pot with the butter and half of the oil. Add the remaining oil on the top of the onions and cover with a lid. Cook over medium low heat for 15 minutes or so. At this point the onions will have started to wilt a bit. Take the lid off, give them a stir and cook for another 15 minutes at medium heat.

Next add the brown sugar and a teaspoon of salt and pepper, stirring well. Cook the onions for another 30 minutes or so, stirring as the onions begin to darken and caramelize. Adjust the heat if your onions are browning too quickly or not browning enough.

Once the onions have turned a beautiful golden brown, add the wine and let it cook down by about half, deglazing the pan. Then add the beef broth and herbs, bring to a simmer and cook on medium low for 30 minutes. Remove the bay leaves and thyme sprigs and taste, seasoning with more salt and pepper if necessary.

Now carefully blend the soup either with an emulsion blender or in a stand blender until smooth. Now swirl in the the sherry while you make the cheese toasts if using. Serve hot, topped with cheese toasts!