French Onion Soup with Beer

Name: French Onion Soup with Beer
Prep time: 20 min
Cook time: 55 min
Total time: 1 h 15 min
Cuisine: European, Mediterranean, French
Ingredients: 10 cups (1.6 kg) thinly sliced onions (about 10 medium onions); 1/4 cup (55 g) butter; 1 tbsp unbleached all-purpose flour; 1 bottle (12 oz/341 ml) local lager; 1 tbsp (15 ml) Dijon mustard; 4 cups (1 litre) chicken broth; 6 thick slices baguette, sliced diagonally and toasted; 2 cups (200 g) grated cheddar cheese