In a large pot, sauté onions in butter over medium heat for at least 20 minutes, until very soft. Keep the lid on the pot to ensure that the onions don’t burn. Remove lid from pot and stir in the brown sugar. Keeping the lid off the pot, sauté for an additional 10 minutes. When done, remove half the onions from the pot and set aside.

Sprinkle the flour over the remaining onions and mix well to combine. Pour the water into the pot. Using an immersion blender, blend together with the onions. Return the remaining onions to the pot, along with the tomatoes, sherry, soy sauce, onion soup mix, and seasonings.

Bring the soup to a low boil, then lower the heat and simmer for 25–30 minutes.

To serve, place a handful of salad croutons into a bowl. Sprinkle 2–3 tablespoons of shredded cheese over croutons. Ladle hot soup over the cheese and croutons.