Preparation

Make the broth

• Put the oil in a large pot over medium-high heat. Season the chicken lightly with 1 tsp. salt. Add the chicken thighs to the pan, skin side down (they should fit in one tight layer), and cook until browned, about 6 minutes. Flip—don’t worry if some skin sticks here and there—and cook for another minute.
• Add the carrots, celery, onion, garlic, parsley, thyme, and bay leaf. Add 2 quarts of water, and bring to a gentle boil. Cover partially, reduce to a simmer, and cook, stirring occasionally, until the chicken is cooked through and tender, about 1 hour.
• Transfer the chicken to a rimmed baking sheet and pour the broth through a fine-mesh strainer set over another container. Discard the vegetables.
• When the chicken is cool enough to handle, remove and discard the skin. Take the meat off the bones, and shred or cut it into bite-size pieces. (The broth may be made up to 2 days ahead. Refrigerate the chicken and the broth in separate covered containers.)

Make the soup

• Skim off the layer of fat on top of the broth. Put the broth in a large pot over medium heat. After it has warmed through, add 1-1/2 tsp. salt. Add the cabbage, broccoli, cauliflower, carrot, and onion, and bring to a boil. Reduce to a simmer and cook until just tender, about 5 minutes.
• Meanwhile, put the butter in a large nonstick skillet over medium heat. Add the bread cubes, stir to coat with the butter, season with 1/4 tsp. salt, and cook, stirring occasionally and adding more butter if needed, until crisp and browned on most sides, about 8 minutes.
• Add the chicken to the soup, and cook until heated through, about 3 minutes. Season to taste with salt.
• Divide the soup among serving bowls. If you like, add a splash of vinegar to each bowl. Divide the croutons among the bowls, garnish with caraway seeds, and serve.