Add the chicken, onion, diced tomatoes with their juice, paprika, garlic powder, salt, and pepper to the slow cooker.
Stir gently until the chicken is coated.
Cover and cook on LOW for 5–6 hours or on HIGH for 3–4 hours, until the chicken is tender and cooked through.
Turn off the slow cooker and let it sit uncovered for a few minutes. For a thicker sauce, instead leave the lid off on LOW for the last 15–20 minutes of cooking.
Place the sour cream in a small bowl and whisk in a ladle of the hot sauce until smooth.
Stir the sour cream mixture into the slow cooker until the sauce is creamy.
Taste and adjust the salt and pepper. Sprinkle with fresh parsley and serve.