French Onion Soup

Name: French Onion Soup
Prep time: 10 min
Cook time: 30 min
Total time: 40 min
Cuisine: European, Mediterranean, French
Ingredients: 1 tablespoon olive oil; 4 tablespoons salted butter; 3 onions, sliced; 4 pieces of star anise; 2 tablespoons red wine vinegar; 1 cup red wine (medium-full bodied); 900 milliliters beef stock; Salt and pepper, to taste; Baguette, sliced into 4 one inch-thick slices; 300 grams gruyere cheese, grated