Set a large pot to medium heat. Add the butter and olive oil and heat until the butter has melted. Add the onions and star anise and cook until the onions have softened.

Add the red wine vinegar and stir to combine. Add half of the red wine and cook until evaporated. Remove the star anise. Add the beef stock and remaining red wine. Bring the heat to high and once boiling, reduce to low heat and simmer uncovered for 25 minutes. Season with salt and pepper.

Divide the soup between four french onion soup bowls. Place one slice of baguette onto the surface of each bowl of soup and evenly distribute the gruyere cheese on top.

Place the bowls on a baking sheet and broil in your oven for 3-4 minutes. Remove from the oven and enjoy!