Make the broth: Place the chicken and beef into a large stock pot and cover with about 12 cups of cold water. Turn the heat on to high and bring to a boil. Skim any impurities that rise to the top. Reduce the heat to medium-low and simmer for 1 hour, skimming the broth occasionally.

Make the croutons: Place a large skillet over medium-high heat. Once the pan is hot, add the cubed pieces of bread to the dry pan and toast them for a few minutes, tossing often, until they are dried. Drizzle with oil and sprinkle with salt, and continue to fry the croutons until they are a deep golden brown and crunchy, about 6 minutes. Remove from the heat onto a paper towel-lined plate to cool.

Once the broth has simmered for an hour, add the onion, carrot, celery root, dill, parsley, salt and pepper. If too much water has evaporated, add enough to cover the ingredients. Simmer the broth for another hour, until the meat is fully cooked through. 

Reserve the chicken and meat on a cutting board. Strain the broth, discarding the vegetables. Shred the reserved meat, and add it back into the pot, along with the strained broth. Taste and adjust the seasoning if needed.

To serve, add a few ladles of hot broth and meat into a bowl, and top off with a handful of  croutons.