Melt butter in large soup kettle or Dutch oven over medium-high heat; add sliced onions and 1/2 teaspoon salt; 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,
approximately 30 to 35 minutes.
Stir in chicken and beef broths, red wine, parsley, thyme, and bay leaf, scraping pot bottom with wooden spoon to loosen browned bits; bring to simmer. Simmer to blend flavors, approximately 30 minutes.
Remove herbs and discard. Stir in balsamic vinegar and adjust seasonings with salt and pepper.
NOTE: At this point, the soup can be cooled to room temperature and refrigerated in an airtight container up to 2 days. To use, return to simmer before finishing soup with croutons and cheese.
To serve, adjust oven rack to upper middle position; heat broiler.
Set serving bowls (use fairly deep, oven- and broiler-safe serving bowls) on baking sheet and fill each with about 1 1/2 cups soup. Top each bowl with two baguette slices and divide Swiss cheese slices, laying them in a single layer on top of the bread. Sprinkle each with about 2 tablespoons grated Asiago cheese and broil until well browned and bubbly, about 10 minutes.
Tip: For an onion soup not
to collapse, the soup bowl has to be filled to the rim with the onion soup
and the bread. The cheese layer should cover the whole surface, so it will
stick to the sides and corm a crust that holds its shape and doesn't sink.
Remove from oven and serve immediately.
Makes 6 servings.
