In a large bowl or shallow dish combine 3/4 cup Bloody Mary mix, 1 tablespoon Worcestershire sauce, pepperoncini juice and 1/2 teaspoon celery salt. Add beef; turn to coat. Cover and refrigerate 4-5 hours, turning occasionally., Drain beef, discarding marinade. Select saute setting on a 6-qt. electric pressure cooker. Adjust for medium heat; add oil. When oil is hot, brown roast on all sides; remove from cooker. Add onions; cook and stir until lightly browned, 4-5 minutes. Remove from cooker. Add stock to pressure cooker. Cook 1 minute, stirring to loosen browned bits from pan. Stir in tomato paste and the remaining 1/4 cup Bloody Mary mix and 1 tablespoon Worcestershire. Press cancel., Return roast and onions to cooker. Add celery, pepperoncini, garlic, pepper and remaining 1/2 teaspoon celery salt. Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 45 minutes. Allow pressure to release naturally for 10 minutes; quick-release any remaining pressure. Remove roast to a serving platter; tent with foil. Let stand 10 minutes before serving. If desired, thicken cooking juices to serve with roast.