Rassolnik with Rice (Russian Pickle Soup)

Ingredients

• 1.5 pounds bone-in, skinless chicken thighs
• 3 quarts water, or as needed
• 2 medium onions
• 3 large pickles
• 0.125 teaspoon whole black peppercorns
• 1 large bay leaf
• 1 tablespoon vegetable oil
• 2 tablespoons tomato paste, or more to taste
• 0.75 cup uncooked white rice
• 2 large potatoes, peeled and cubed
• 0.5 teaspoon salt, or more to taste
• 0.25 cup sour cream, or to taste