Step 1

Melt butter in a deep stock pot or Dutch oven over medium-high heat.

Step 2

Add onions and cook, stirring occasionally, until tender and browned, about 15-20 minutes.

Step 3

While onions are cooking, heat the oven broiler.

Step 4

Add beef broth, balsamic vinegar, thyme, salt, and pepper and bring to a simmer.

Step 5

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

Step 6

Line a baking sheet with tin foil and place the baguette slices down flat. Use 2-3 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.

Step 7

Ladle the onion soup evenly into 4 oven-safe serving bowls, starting with the onions and then with the liquid.

Step 8

Place 2-3 baguette slices atop each bowl, followed by a slice of provolone cheese.

Step 9

Place bowls on the baking sheet you used for the baguettes and broil until the cheese is bubbly and golden, about 5 minutes.