French Onion Soup with Gruyere and Cognac

Boulestin doesn't call it French onion soup, it's simply Soupe a l'Oignon, and apart from the number of onions, he doesn't give quantities. He also omits the cognac, which I'm adding towards the end. For four small serves, I'm guessing 3 cups of water.

French Onion Soup with Gruyere and Cognac

2 large onions, finely sliced
50g butter
50 ml cognac
50g Gruyere cheese
baguette
salt, pepper

Saute the onions in butter until golden brown. Add 3 cups hot water plus a little extra to allow for reduction; season with salt and pepper. Simmer, uncovered, for 30 minutes, adding the cognac about 5 minutes before the end to allow it to cook off. Have ready a few thin slices of baguette, dried in the oven. Pour your soup over these in the soup tureen, sprinkle with grated cheese and quickly brown under the grill.