Step 1:

In a slow cooker (5- to 8-quart capacity), arrange the onion, carrots, celery, garlic, and bay leaf. Remove the skin from half of the chicken thighs and discard it or reserve it for another use. Place the chicken thighs in the slow cooker, then pour in the chicken broth. Generously season with black pepper. Add ½ teaspoon of salt if using low-sodium broth, or 1 teaspoon for unsalted homemade stock. If the broth is pre-salted, omit additional salt at this stage. Cover the slow cooker and cook on low for 4 to 6 hours. For added convenience, the slow cooker can be switched to warm after 6 hours, keeping the soup ready for up to 2 additional hours before the chicken dries out.

Step 2:

Using a ladle or spoon, skim off any foam or fat visible on the soup's surface. Remove the chicken and vegetables with tongs or a slotted spoon and transfer them to a bowl. Strain the broth and discard the vegetables unless they still retain flavor. Let the chicken cool briefly. Return the strained broth to the slow cooker and increase the heat to high. Add the cooked rice, cover, and heat for about 10 minutes until the soup is steaming or lightly bubbling at the edges.

Step 3:

In a medium mixing bowl, whisk together the lemon juice, egg yolks, and sour cream until the mixture is smooth and creamy. Gradually incorporate about 2 cups of hot broth into the lemon mixture, whisking constantly to prevent curdling. Pour the tempered mixture into the slow cooker and whisk for about a minute until the soup thickens enough to coat the back of a spoon. Shred the chicken meat, discarding the bones and skin, and add it back to the soup. Stir in the chopped dill and taste, adjusting with additional salt, lemon juice, or pepper as needed. If the soup requires additional heating, cover and cook on high for 10 more minutes before serving.