In a large bowl, combine the onions, sugar, and salt and stir to mix well. In a heavy large saucepan over medium-high heat, heat the butter until almost smoking. Add the onion mixture and stir until onions begin to brown, about 3 minutes. Reduce heat to low and cook until the onions are soft and golden brown, stirring occasionally. Stir in the white wine and sherry. Increase the heat to medium-high and add the strained chicken stock. Bring to a boil, reduce heat to medium and cook to reduce to about 6 cups. Mix together flour and water in separate bowl. Stir flour mixture into saucespan and return to a boil. Simmer 5 to 10 minutes. Season with salt and pepper and pour into wide soup bowls. Add the croutons and sprinkle with cheese.