This recipe comes from Julia Child's The Way to Cook, page 19. It's a great use for a homemade broth from a left over pot roast bone. I always wish I'd doubled the recipe because it reduces quite a lot.
1/2 cup butter
8 cups or 2 1/2 lbs onions (3 huge ones)
1/2 tsp rapadura or sugar
1/2 tsp sea salt
2 TB flour
2 1/2 qts homemade beef stock
4-5 TB Cognac
1 cup dry white French vermouth
Thinly slice onions. Melt butter in pan over medium heat. Add onions and cover for 10 minutes or until translucent. Add salt and sugar, raise to med-high, and let onions brown, stirring frequently, 45-60 minutes or until dark walnut colored. Sprinkle in flour and stir 3-4 minutes. Remove from heat and whisk in 2 cups of hot stock. Bring to a simmer and add the rest of the stock, brandy and vermouth. Cover loosely and simmer 1 1/2-2 hours, adding a little water if necessary.
Serve with french bread or gratine by topping soup bowls with toasted french bread slices and Swiss or Parmesan cheese and then putting under the broiler until cheese starts to brown. May substitute Cognac with peach juice and vermouth with apple cider or substitute both with apple juice mixed with a little bit of lemon juice and water. I do the later usually.