Sautée onions in butter for about 3 minutes. Sprinkle brown sugar over and cook a further 5 minutes. Add in red wine and tarrragon and cook another 10-15 minutes until onions are golden coloured and soft.

Preheat broiler while stock is simmering below.

Heat stock and champagne in a sauce pan. Add the onion mixture and bay leaf. Simmer about 12 minutes. Remove bay leaf. Season to taste with salt and pepper. ( I did not need any).

Spread baguette slices on a baking sheet. Toast under broiler until just golden brown. (4 minutes for my oven). Turn over and toast until golden on the other side. Remove from oven and sprinkle cheese over the baguette slices. Return to oven til cheese is melted and bubbly.

When ready to serve, divide soup between 6 bowls and float cheesy crouton in middle of bowl.