In a large saucepan over medium heat, add the olive oil and onions and saute for 20-25 minutes or until the onions are soft and translucent. Add the sugar about half way through. Add a little broth while you are sautéing the onions I find that it helps caramelize them quicker and infuses them with more delicious pho broth
Add the wine and cook for a few minutes then add the pho broth. Bring up to a boil and then simmer for 45 mins to 1 hour. You want the onions to be soft and for the broth to have reduced slightly. This is going to give you a much richer broth
Preheat the oven to 350 degrees Fahrenheit and on a baking pan covered with baking paper, add the French bread and top each with 1oz of the Gruyere Cheese. Toast until melted and bubbly
Divide the soup between each bowl and top each with the bread and cheese slice and cilantro and chilli peppers if using
