In pressure cooker, brown brisket in hot oil on all sides evenly, turning with long-handled fork or tongs. Remove meat and set aside. Saute onion, garlic, and sliced carrots in hot oil for 3 minutes, stirring and scraping bottom of cooker to loosen any browned particles of meat. Stir in broth, lemon juice, tomato paste, parsley and seasonings. Place rack in cooker; add meat. Secure lid. Over high heat, bring cooker to high pressure. Reduce heat to medium and cook for 50-55 minutes. Release steam according to manufacturer's directions. Remove lid. Stir potatoes and carrots into meat mixture. Secure lid. Over high heat, bring cooker to high pressure. Reduce heat to medium and cook for 9 minutes. Release steam according to manufacturer's directions. Remove lid. Transfer meat and vegetables to serving platter. Loosely cover to retain heat. Skim all but approximately 2 T. liquid from cooker; strain this liquid through fine sieve and set aside. Heat cooker over low heat; stir in flour to make a smooth paste. Add sieved liquid; stir continuously until the sauce is smooth and thickened -about 5-7 minutes. Remove skillet from heat; whisk in sour cream and horseradish until well blended. Slice brisket. Serve sauce with brisket and vegetables. NOTE: I have made this dish many times without the horseradish sauce. It is very flavorful without the sauce, and excellent served with a simple salad. VARIATION: Instead of making the horseradish sauce, simply add the following to the cooker with the tomato paste: 1 t. oregano, 1 t. thyme, 1 t. basil, half t. rosemary -OR1 T. Italian seasoning. You may also substitute 1 c. red wine (do not use cooking wine; use a nice table wine for this purpose) or beer for 1 c. of the beef broth.