The caramelization of the onions is the most important part of the entire recipe– don’t rush it. (And keep your eye on them to make sure they don’t burn!)
The quality of beef stock and chicken stock you use will either make or break this French onion soup recipe. I HIGHLY recommend you use homemade broth for this.
If you don’t want to use wine, you can substitute an extra cup of broth instead. But the wine really brings a beautiful depth of flavor to this recipe.
I have made this recipe without using flour, and it will still work. So if you are gluten-free, that is definitely an option. The addition of the flour just provides a slightly creamier texture, in my opinion.
French onion soup purists always use Gruyere cheese in their recipes. I love Gruyere, but it’s pretty expensive and I never seem to have it on hand. Therefore, I usually opt for swiss, Parmesan, or mozzarella instead.
I suppose you could omit the balsamic vinegar, if you want, but I wouldn’t recommend it, as it adds the most perfect pop of extra flavor.
French onion soup gets even better the second day. If you have leftovers. Which you probably won’t.