To make the onions caramelize relatively quickly and evenly, use a mandolin to slice the onions very thin. The idea is to evaporate as much water as possible, and caramelize the sugars in the onions without burning them
Add the butter to a large dutch over and melt. Add the onions, salt and sugar and cook over medium high heat, stirring occasionally until the onions are wilted and just starting to turn brown (about 20 30 minutes). Turn down the heat and continue caramelizing, stirring frequently to prevent burning (about 30 40 minutes)
When the onions are sticky, glossy, and a medium brown, add the sherry. Deglaze the pan by scrapping up all the browned bits from the bottom of the pot and reduce again, until the onions are sticky, glossy and dark brown (about 10 minutes). Add the beef stock, demi glace, marjoram, bay leaf, then salt and pepper When salting, keep in mind the soup will reduce a bit when you're melting the cheese on top, so don'make it too salty. Let this simmer for about minutes for all the flavors to meld to taste.
Ladle the onion soup into broiler safe ramekins, crocks, or mini cocottes (there should be enough soup for 4-6 individual servings depending on the size of your bowls). Don'fill the containers all the way. Top with the soup with slices of toasted bread and push them into the soup. Top the soup with a generous helping of cheese.
Place your ramekins on a baking sheet. Move the oven rack to the top position, and turn on the boiler. Put the baking sheet in the oven with the French Onion Soup directly below the heating element. Keep an eye on them to prevent burning and broil until the cheese is hot, and bubbly, and lightly browned. Serve right away