In a heavy bottomed pot or Dutch oven, over medium-low heat, add the olive oil and butter. When the butter melts, add the chopped onions with the sugar to help with the caramelizing. Sauté, stirring frequently for 30 to 60 minutes until they turn a rich, brown color. Do not put a lid on the pot as it will cause the onions to ‘steam’ and create liquid in the pan. As the onions begin to color, they may begin sticking to the bottom of the pan. Add a couple of tablespoons of the wine to loosen up the bits and continue the sautéing process. This is a slow process and could take as long as one-hour.

When the onions are a nice rich golden-brown color, add any remaining wine, scraping the bottom to remove the onion bits. Sprinkle and stir the flour over the onions and cook for another two or three minutes. Add the chicken broth, thyme, salt and pepper and bring the pot to a boil. Reduce the heat to simmer and cook for another 15 minutes.

Serve the soup in individual oven-proof bowls with a tablespoon of cognac (if using) drizzled over the top. Add a slice of toasted French bread to each bowl, add one-half cup grated cheese to each slice of toast and set the bowls on a baking sheet. With the oven set on broil, heat the bowls of soup until the cheese is brown and bubbly.