Slice 6 pieces of bread about 1 inch thick, and toast them at 250 F. for 15-20 minutes. Grate 1 pound of Gruyere and chop 2 large red onions into chunky pieces.
Add 4 tbsp. of butter to a large pot. Add chopped onions. Cook until soft and slightly browned, stirring occasionally. Add 1 tsp. of thyme and sprinkle with fresh pepper. Mix with onions for 1 minute.
Pour ⅓ cup quality port wine or sherry into the onions and stir. Allow to reduce and caramelize for 5 minutes.
Once bread is done toasting, remove them and raise oven temperature to 450 F. Pour 6 cups of beef broth into the onions. Turn heat to high and simmer for 15 minutes. Sprinkle with pepper and 1 tsp. salt.
Pour soup into bowls and place toasted bread on top of each bowl of soup.
Add a nice big sprinkle of cheese. Place carefully in the oven for 10 minutes, or until cheese is crispy around the edges.
Remove from oven carefully and serve.