Carefully wash all the vegetables and beef bones. For the stock, you won't need to peel anything except celeriac. Measure all the rest soup components to make your work smoother.
You won't need to peel your carrots, as this way they will bring more flavor. Cut them into approximately 5-cm pieces.
Same way, don't peel the onions reserved for the stock. Cut them into quarters.
Cut the garlic's roots and divide it into two or three parts, there's no precise formulas for that case.
Cut your leeks into similar 5-cm slices.
Transfer the marrow bones and previously cut carrots, onions and garlic into the baking sheet. Roast this mixture for 40-60 minutes at 230°C, it should become deeply brown.
Once the beef stock base is ready, start scraping all the roasted vegetables and bones into the pot, don't forget any juices.
After you transferred the mixture, you may use some water to wash the leftovers in the baking sheet. Pour the rest of water into the pot.
Peel and chop your celeriac, add it together with bay leaves and whole peppercorns into the stock base. At the same time, pour the cider. Bring your beef stock to a boil and then reduce the heat. Simmer for eight hours or even more, if you want to get all the aroma.
Let the beef bone stock slightly cool down before filtering it.
Discard all large parts and then filter your stock with a fine sieve, nothing, except the liquid, should be left.
This time peel your onions and cut them into regular half-rings, but don't try to make them too thin.
Preheat the sunflower oil and combine with 2/3 of butter afterwards, keep the heat medium. Once the butter is melt, transfer the onions. Constantly stir while cooking for 15 minutes, the onion segments should become soft and slightly transparent.
Season the softened onions with kosher salt and sugar. You need them caramelized, so reduce the heat and keep mixing from time to time for 45 minutes.
Pour your cider and increase the heat until all the liquid is evaporated, that takes approximately ten minutes.
Carefully pour your beef bone stock. If needed, add some water. Bring this French onion soup to a boil and then simmer it for twenty minutres.
Sprinkle some dried herbs over your onion soup.
Once the soup is ready, mix the left butter into it.
Cut the bread into individual slices.
Toast your bread in the oven for a couple of minutes.
Peel one garlic clove. Rub your toasts with it.
Get the individual ceramic bowl and pour some brandy into it.
Ladle the onion soup into your bowl.
Grate all your Gruyere cheese and divide it into individual servings.
Put the garlic toast into the soup.
Sprinkle some cheese over your onion soup, especially the toasted bread.
Place into the oven for several minutes until the cheese is melt. Alternatively, you may simply place the toast with grated cheese into the bowl right after the brandy and pour your soup over. This way it will also taste great.
Already see your Gruyere melt? Take it out and serve immediately. You won't regret all the time spent after you try even the first spoonful!