This is what I make when I have tons of onions, we ate it in Montmatre a few years ago - but named it 'cheese soup' because the French do like to put huge amounts of cheese sliced baguette on top - yum!
50g/2oz/1/4 cup butter
3 large onions, sliced
10ml/2 teaspoons plain (all purpose) flour
1 litre/1 3/4 pints/4 cups vegetable (or chicken) stock
60ml/4 tablespoons white wine OR 30ml/2 tablespoons dry sherry
4/6 slices crusty white bread
150g/5oz/1 1/4 cups Gruyere or Emmenthal cheese
Salt & pepper
Slice the onions and pop them in a big saucepan with the butter - don't be tempted to use margarine it really doesn't work as well.
Cook the onions over a low heat for about 10 to 15 minutes, stirring occasionally, until they are lightly browned. Don't rush this stage!
Stir in the flour and continue to cook, stirring continuously until the flours turns a light sandy colour.
Pour in the stock and the alcohol. I have made this without the alcohol and it works very well but you might want to substitute by putting in a few dashes of Worcestershire sauce instead.
Bring to the boil, stirring continuously.
Season with salt and pepper, cover and simmer for 15 minutes.
To serve I find it easiest to scoop out the onions with a slotted spoon into serving bowls then top up with the liquour.
Toast the slices of bread under the grill then sprinkle the cheese on top. Return to the grill and melt the cheese. Place the cheesy toasts onto of the soup once it is served into bowls.