Put the chicken in the pressure cooker; sprinkle the onion on top. In a small bowl, combine flour and stock until smooth. Whisk in the tomato paste, garlic, paprika, salt, thyme and Tabasco. Pour over the chicken.
Close cover securely; place pressure regulator on vent pipe. Bring cooker to low pressure over high heat. Reduce heat to medium-high; cook for 12 minutes. (Pressure regulator should maintain a slow steady rocking motion or release of steam; adjust heat if needed.)
Remove from the heat. Immediately cool according to manufacturer's directions until pressure is completely reduced. Remove chicken and keep warm. Stir sour cream into cooking juices; serve over chicken.