Cut off the stem and root ends of the onions, then halve then lengthwise, and peel away the dried skin. Cut the onions into thin slices. It’s important to have all the onion slices roughly the same size so they cook evenly.
Cook the pancetta fully in a dutch oven. Once cooked, remove the pancetta using a slotted spoon from the dutch oven and reserve for later. You’ll have some delicious pancetta oil at the bottom of the pot and add to that your unsalted butter. Add sliced onions, salt, stirring to coat onions thoroughly with the butter/oil. Cook, stirring frequently, until onions are reduced and syrupy and inside of pot is coated with very deep brown crust, approximately 2 hours.
Stir in chicken and beef broths, wine, and be sure to scrape the bottom of the pot with a wooden spoon to scrape off all those delicious brown bits. Add parsley, thyme, and bay leaf and cooked pancetta then bring to a simmer. Simmer to blend flavors, approximately 30 minutes. Remove herbs and discard. Stir in balsamic vinegar and adjust seasonings with garlic, salt and pepper to taste. 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.
Preheat oven to 400 degrees F. While the soup simmers, toast your baguette. Lightly brush the baguette pieces with olive oil and arrange the baguette slices in single layer on baking sheet and bake until the bread is dry, crisp, and golden at edges, about 10 minutes. Set aside until ready to use.
When ready to serve the onion soup, adjust oven rack to upper middle position; heat the broiler. Set onion soup bowls on baking sheet and fill each with about 1 ½ cups soup. Top each bowl with toasted baguette slices that cover the top of the bowl without overlapping the bread. Over the bread lay the mozzarella in a single layer. Besure to slightly overlap the cheese over the edge of the bowl. This acts as a delicious little safety net that keeps the grated cheese from falling into the soup abyss
Sprinkle the top of each with about 2 tablespoons grated Asiago cheese and 2 tablespoons Gruyere cheese. Place the bowls into the oven, about 5 to 6 inches from the broiler and let broil until cheese is brown and bubbly, about 10 minutes. Remove from oven and serve immediately.