Melt the butter and olive oil in a pot over medium heat. Add the onions and cook until the onions have softened and are transparent in color, around 18 to 20 minutes.
Add salt, pepper, and sugar. Stirring constantly, cook for 30 more minutes or until the onions are caramelized and have a deeper golden color.
Increase the heat to high and add the white wine. Cook for 12 more minutes or until the wine has reduced almost completely.
Add the broth, rosemary and thyme and bring to a simmer. Stirring occasionally, cook until the broth has thickened and the flavors have bloomed, around 20 more minutes.
Serve the soup into bowls. Top with the toasted baguette and a sprinkle of cheese and take to the broiler. Broil for around 7 minutes or until the cheese is melted and bubbly.
Serve immediately topped with fresh herbs or a crack of pepper.
Use a good quality broth! The secret for making this soup restaurant quality is using a delicious broth. If you want to store your soup, skip the toppings and refrigerate for up to 3 days. Once you are ready to serve it, heat your soup on the stove and finish with fresh toasted bread and cheese. The result is just as good as the day you made it. If you don’t have oven proof bowls, you can toast the bread with the cheese and then just top the soup with it.