In a large, heavy bottomed casserole or skillet, heat oil over medium high heat.
Add as many chicken pieces that will fit without crowding. Sauté until browned on both sides, about 8 minutes. Remove to a paper towels to drain and repeat with remaining pieces.
Reduce heat to low and add onions, carrots, and celery. Cook, stirring occasionally, until the onions turn clear, about 10 minutes.
Add stock or water and chicken pieces. Sprinkle with paprika and salt. Bring liquid to a boil, reduce heat, and cover. Simmer until juices run clear when thigh is pierced with a fork, about 30 minutes.
Line a colander(or strainer) with a paper towel or coffee filter. Place in a container so liquid can drip through. Pour yogurt into strainer and let drain. If using sour cream, skip this step.
With tongs, remove chicken from casserole to a heated platter.
Skim any fat from the liquid in casserole. Reduce stock to about 1/2 cup, boiling for 3-4 minutes.
In small bowl place drained yogurt, discarding paper towel. Add several tablespoons cooking liquid and whisk. Reduce the heat under the casserole to low and whisk yogurt into the pan. Cook for 1 minute, then pour sauce over the chicken, sprinkle with scallions and parsley, serve immediately. Enjoy!