Slightly modified from Epicurious.com and Bon Appetit
Like I mentioned in this post...I got bored stirring the onions and had fun playing with flour, salt, pepper, and thyme. You should do the same and make it your own.
1/4 cup (1/2 stick) butter
6 onions (about 3 pounds), sliced
6 garlic cloves, sliced
1/2 cup red wine (I used a Beaujolais)
4 cups good beef or chicken broth
2 cups water
2-3 teaspoons Dijon mustard
or a mixture of Swiss and Parmesan cheeses, grated
Melt butter in heavy large saucepan over medium heat. Add onions and garlic and sauté until very tender and brown, about 45 minutes. Add wine and simmer until reduced to glaze, about 3 minutes. Stir in chicken or beef broth, water, and mustard. Simmer 20 minutes. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Refrigerate. Return soup to simmer before continuing.)
Preheat broiler. Now, you can either: ladle soup into broilerproof bowls and top each with slice of toast and grated cheeses, broiling until cheeses melt and bubble, OR you can be lazy (or just lacking in boiler-proof bowls like me) and just toast the bread and cheese and then put it on top of the soup.
As for my next tweaking of the recipe, I think next time I'll do mostly chicken or a combo of chicken and beef broth, and use a white wine instead. It's fun to play with soup.