Melt the butter in a large saucepan. Add the onions & toss to coat. Lay a piece of wax paper, cut to fit tightly over the onions. Cook over very low heat, without stirring for 20-30 minutes or until the onions are soft and golden brown. Add the stock, wine, thyme, nutmeg, salt & pepper and bring to a boil over medium-high heat. Reduce heat, cover & simmer for 45 minutes.
Preheat oven to 350°. Toast bread slices in oven until dry, but not brown, approx. 10-15 minutes.
To serve, divide soup into 6 heat-proof bowls, top with slices of bread and sprinkle with cheese. Place bowls under broiler until cheese is golden & bubbling. Bowls are very hot – be careful when serving!
Serves 6