Peel the carrots, leek, and celery and cut them into coarse pieces. Then cut the onion in half, but do not peel it since the peel gives the soup a lovely color.
Now fry the onion halves briefly in the pot with the cut side down. Then add the rest of the vegetables and bones and fill the pot with water.
Add spices and apple cider vinegar, the acidity of which helps to extract collagen and minerals from the bones better.
Bring to a boil, then reduce to a simmer and cover. Simmer for 12 hours. The further it reduces, the more intense the flavor will be, and the more collagen will be extracted. Optionally, you can add water and get a thinner broth. Alternatively, you can use an Instant Pot as well.
Strain broth and enjoy or store.