Thinly slice onions. Melt butter in Dutch Oven over medium heat.
Add onions and 1/2 teaspoon salt, stir and cover for 5 minutes.
Remove lid and let onions caramelize until golden brown over medium heat, and be sure to give it a stir every now and then! This process can take anywhere from 45-60 minutes. You can turn the heat down a bit if they are cooking too quickly, and even add a couple of tablespoons beef stock or water if you feel that they are burning.
Warm beef stock in saucepan.
Add the wine and the sherry to the onion mixture, give a stir and bring to a boil. Add the flour, stir, and let thicken 1-2 minutes.
Add the warmed broth, salt and pepper to the onion mixture, and boil uncovered for @ 10 minutes.
While the soup is boiling, grate your cheese.
Heat your broiler. Place oven safe casserole/onion soup bowls on a baking sheet, add soup, one slice French baguette and top with cheese.
Broil 1-2 minutes, watching carefully. Serve immediately.