Combine soy sauce, sherry, onion, olive oil, brown sugar, rosemary, garlic, and pepper in a bowl and mix well. Pierce flank steak throughout using a large fork, flip, and repeat. Place steak in a plastic zip-top bag and pour in marinade to cover. Marinate in the refrigerator, 8 hours to overnight.

Combine sour cream, green onions, horseradish, salt, and pepper in a bowl. Cover and refrigerate sauce until ready to use, up to 24 hours.

Preheat an outdoor grill for high heat and lightly oil the grate.

Drain steak and pour marinade into a small saucepan. Bring marinade to a boil over high heat and boil for 1 minute. Remove from heat. Season steak with pepper.

Cook steak on the preheated grill to desired doneness, basting occasionally with marinade, about 5 minutes per side for medium doneness. An instant-read thermometer inserted into the center should read 140°F (60°C).