Soak the dried okra. Place the dried okra in a heatproof bowl. Add the soaking water while hot but no longer actively boiling, then add the soaking lemon juice. Keep the pods submerged and soak for 30 minutes.
Drain and remove the string. Drain the okra into a strainer and discard all soaking liquid. Rinse briefly under cool running water and drain gently without squeezing the softened pods. If the okra was threaded, carefully slide every pod off and remove all pieces of string.
Prepare the vegetables and chicken. Cut the peeled carrots, celeriac, and parsley root into large pieces. Halve the peeled onion and lightly crush the peeled garlic. Leave the boneless, skinless chicken thighs whole. Do not rinse raw chicken; keep its utensils separate from herbs and ready-to-eat foods, and wash hands and clean surfaces after handling.
Start the broth. Place the chicken, onion, carrots, celeriac, parsley root, and garlic in a suitable soup pot. Add the bay leaf, broth salt, and cold broth water. Heat slowly over medium-high heat, skimming foam from the surface.
Simmer and check the chicken. As the broth reaches a boil, reduce the heat and set the lid slightly ajar. Simmer gently for about 40 minutes, avoiding a rolling boil. Check the center of the thickest chicken piece with a clean thermometer: it must reach at least 165°F (74°C). Continue cooking if needed; the vegetables should also be tender.
Reserve the broth. Lift the cooked chicken and the carrots, celeriac, and parsley root onto a clean plate. Discard the onion, garlic, and bay leaf. Carefully strain the broth into a large heatproof bowl and keep it hot. Briefly rinse and dry the soup pot.
Shred and dice. Let the cooked chicken cool only enough to handle safely, then shred with two forks into bite-size pieces. Dice the cooked carrots, celeriac, and parsley root into roughly 1/4-inch (5–8 mm) pieces. Continue the preparation promptly.
Make a pale roux. Melt the butter in the cleaned pot over low to medium heat. Whisk in all the flour and cook, stirring, for about 2 minutes. Keep the mixture pale; do not let it brown or scorch.
Whisk in the broth. Add a small portion of the hot reserved broth while whisking continuously until smooth. Add another portion and whisk again. Continue gradually with the rest of the broth, then bring the smooth mixture back to a gentle simmer, stirring along the bottom.
Cook the soaked okra. Add the drained okra to the thickened broth. Simmer gently for about 15 minutes, stirring carefully from time to time to prevent sticking. The pods should soften while retaining their shape. If still firm, continue gently and check again rather than assuming soaking alone cooked them.
Return the chicken and roots. Add the shredded chicken and diced carrots, celeriac, and parsley root. Stir in the black pepper and heat gently for about 5 minutes, until everything is hot throughout. Avoid vigorous boiling or crushing the okra.
Finish with lemon and parsley. Stir in the finishing lemon juice. Add about two-thirds of the chopped parsley and reserve the rest for serving. Taste the broth before making any further seasoning adjustment. The soup should be lightly thickened and gently tangy, not gluey or sharply sour.
Serve and store safely. Divide the soup evenly into the listed servings and sprinkle with the reserved parsley. Refrigerate leftovers promptly in shallow containers within 2 hours, or within 1 hour above 90°F (32°C). Keep at 40°F (4°C) or below, use within 2 days, and reheat throughout to 165°F (74°C).