French Onion Soup

This is from America's Test Kitchen.

2 Tbs unsalted butter
5 medium red onions (about 3 pounds) sliced thin
table salt
6 cups low-sodium chicken broth
1 3/4 cups low sodium beef broth
1/4 cup dry red wine
2 sprigs fresh parsley leaves
1 sprig fresh thyme leaves
1 bay leaf
1 tablespoon balsamic vinegar
ground pepper
1 baguette, cut on the bias into 3/4 inch slices (2 slices per serving)
4 1/2 ounces swiss cheese, sliced 1/16 inch thick
1 1/2 ounces grated Asiago cheese (about 3/4 cup)

Melt butter in large soup kettle or dutch oven over medium high heat.  Add sliced onions, and 1/2 teaspoon salt and stir to coat onions thoroughly with butter.  Cook, stirring frequently, until onions are reduced and syrupy and inside of pot is coated with very deep brown crust, 30-35.  Stir in the chicken and beef broths, red wine, parsley thyme, and bay leaf, scraping pot bottom with wooden spoon to loosen browned bits, and bring to simmer.  Simmer to blend flavors, about 20 minutes and discard herbs.  Stir in balsamic vinegar and adjust seasonings with salt and pepper.  (Can be cooled to room temperature and refrigerated in airtight container up to 2 days; return to simmer before finishing soup with croutons and cheese).

For the crusts, adjust an oven rack to the upper-middle position and heat the oven to 350 degrees.  Spread the bread out on a rimmed baking sheet and bake, flipping once, until lightly browned, about 15 minutes.  Remove the bread from the oven and turn the oven to broil.

Set heat-safe soup bowls or crocks on rimmed baking sheet and fill each with about 1 1/2 cups soup.  Top each bowl with two toasted baguette slices and divide Swiss cheese slices, laying them in a single layer if possible on bread.  Sprinkle each serving with about 2 Tbs grated Asiago Cheese and broil until well browned and bubbly, 7-10 minutes.  cool 5 minutes and serve.