French onion soup: peel and cut each onion in half and then thinly slice. You should be left with a whole bunch of half-circle slices of onion. Add 2 tablespoons of butter to a medium pot over medium heat. Once the butter is melted and foamy, add all your onions. You may want to add the onion slices in batches, allowing each batch to cook and soften a bit before adding the next batch.
Once all the onions have been added, sprinkle in 1 teaspoon of salt and stir to incorporate. Allow the onions to cook untouched for about 10 minutes. After that time, stir the onions and continue cooking and stirring occasionally until they are dark brown. This process should take about 45 minutes. If you ever notice that the onions are sticking to the bottom of the pot and starting to burn, you can add a tablespoon of water and stir until the liquid boils off. This water will cool everything off and allow the onions to caramelize without burning.
Once your onions are dark brown, tie the thyme, bay leaf, and parsley together with a long piece of string or butcher's twine. This is so the herbs can float in the simmering soup, and you are able to retrieve them later.
Add 1 cup of white wine, 32 ounces of beef broth, and the tied-up herbs. Tie the other end of the string with the herbs to the handle of the pot so you can pull them out easily.
Simmer the soup for 20 minutes.
After 20 minutes, carefully taste the soup and see if it needs more salt. Add ground black pepper and salt if needed. Pour in a splash of sherry, stir the soup, simmer for another minute and then turn off the heat. Remove and discard the twine and the herbs.
Sandwich assembly: slice your roll if needed.
If your onions and broth are coming straight from the fridge, you need to warm them. You can use a microwave or two pans on the stove. The broth needs to be warm for dipping and warming the onions will help the cheese melt in the middle of the sandwich.
Add half of your cheese to the bottom of the bread roll. Top with the warm onions and then cover the onions with the rest of the cheese. Close the sandwich.
Spread butter on the top of the roll.
Cook the sandwich in a panini press until the bread gets crusty and browned. If you do not have a panini press, you can use a hot cast iron skillet with another skillet on top of the sandwich, pressing it down. This will griddle one side and you will have to flip the sandwich, but it will work just fine.
Cook the sandwich until the bread is turning golden brown and the sandwich is flattened about 5 minutes.
Serve sandwich with a small bowl of the warm au jus for dipping or for enjoying as soup alongside the sandwich.