1. Combine first 3 ingredients in a food processor; process until onion is minced.
2. Place steak in a large heavy-duty zip-top plastic bag. Pour onion mixture over steak; seal bag, and shake until steak is well coated. Marinate in refrigerator 8 hours, turning bag occasionally.
3. Prepare grill.
4. Remove steak from marinade; discard marinade. Place steak on grill rack coated with cooking spray. Cover and grill 10 to 12 minutes or to desired degree of doneness. Let steak stand 5 minutes.
5. Combine sour cream, mayonnaise, and horseradish, stirring well. Cut steak diagonally across grain into thin slices. Garnish with oregano sprigs, if desired. Serve with horseradish mixture.
carbo rating: 8