Preheat your oven to 400 degrees F.

Place the cubed bread on a rimmed baking sheet. Drizzle with olive oil and toss to coat; sprinkle with salt and pepper to taste.

Bake in the preheated oven for about 15 minutes, tossing the cubes about halfway through baking.

Remove from the oven and set aside.

Heat a large saucepan over medium heat. Add the olive oil and butter to the pan.

Once the butter has melted, add the sliced onions and stir, making sure that the onions are coated with the oil and butter.

Cover and reduce the heat to medium-low. Cook for about 10 minutes, stirring occasionally, until the onions are softened and some of the moisture has been released.

Add the Kosher salt, bay leaf, and thyme. Continue to cook uncovered for 15 – 20 minutes, stirring often. You want the onions to be caramelized so they are a nice brown color; be careful not to burn them.

Remove and discard the bay leaf and add the minced garlic, cooking for about 30 seconds.

Add the red wine and brandy to the onions, bring to a simmer, and cook for 2 – 3 minutes.

Stir in the flour and cook for 1 – 2 minutes.

Add the beef broth, Worcestershire sauce, and bullion cube. Stir, making sure that everything is well combined.

Bring to a boil and simmer gently with the lid off for 8 – 10 minutes, stirring occasionally. 

Taste and adjust the with salt and pepper as needed.

Move your oven rack to the top of the oven and turn the oven on to broil.

Ladle the soup into oven-safe French onion soup bowls. Place the croutons on top, covering the surface of the soup.

Divide the grated Gruyere cheese between the two bowls. Place them on a baking sheet and broil for 1 – 2 minutes or until the cheese is melted and slightly browned. Be careful not to burn them.

Serve immediately.