In a large pot, add the butter and olive oil over medium heat. Once the butter is melted, add in the thinly sliced onions. Bring heat to low-medium and cook for 30–40 minutes, stirring consistently, until the onions are caramelized and a deep golden-brown color.

Once the onions are caramelized, add the grated garlic and deglaze the bottom of the pot with wine over low-medium heat. When the wine has reduced by half, add the beef broth, thyme, bay leaves, and salt and pepper. Stir to combine and bring the broth to a boil. Once boiling, reduce to a simmer and let the soup cook, covered, for 40 minutes to 1 hour.

Preheat the oven to broil. Evenly distribute the soup into 4–6 ramekins or oven-safe soup bowls and place them on a baking sheet. Top each bowl with 2–3 slices of baguette, then add 2 tablespoons of gruyère and 2 tablespoons of aged cheddar to each.

Place the baking sheet in the oven and broil for 1–2 minutes, until the cheese is melted and bubbly.

Remove from the oven and top with freshly cracked black pepper, grated parmesan, and fresh thyme.

Serve and enjoy!