French Onion Soup

Melt butter in a deep stock pot or Dutch oven over medium-high heat. Add onions and cook, stirring occasionally, until tender and browned, about 15–20 minutes. (While onions are cooking, heat the oven broiler.) Add beef broth, white wine, balsamic vinegar, Worcestershire sauce, thyme, salt, and pepper and bring to a simmer.

In a small bowl, combine chicken broth with flour, and then add to the onion mixture. Cook for an additional 10 minutes.

Line a baking sheet with tin foil and place baguette slices down flat. Use 2 to 3 slices per person, depending on the size of your serving bowl. Place into oven and broil for 1–2 minutes per side, until golden brown. Remove from oven but leave broiler on.

Ladle the onion soup evenly into oven-safe serving bowls, starting with the onions and then with the liquid.
Place baguette slices atop each bowl, followed by a slice of provolone cheese, and then topped with the shredded Gruyere cheese. Place bowls on the baking sheet you used for the baguettes and broil until the cheese is bubbly and golden, about 5 minutes.