These sandwiches that feed a crowd are gone quickly at Guy's house. For flavor boost, marinate the beef 1 or 2 days. "If you can't wait, just go with it. It won't be over-the-top, but it will be really good," Guy says.
For the Spiced Beef, combine spices for the meat in a resealable 1-gallon plastic bag; shake to mix thoroughly. Add one piece of meat, shake it around in the bag, remove it and repeat with the second piece of meat. Place both pieces of meat in the bag; seal bag. Marinate both pieces as long as you can in the refrigerator (24 to 48 hours is recommended).
For Horseradish Cream, in a medium bowl combine the sour cream, mayonnaise, lemon juice, horseradish, garlic, sea salt, and pepper to taste. Mix thoroughly and refrigerate for at least 4 hours.
Twenty minutes before you plan to grill, remove the meat from the refrigerator and let stand at room temperature. Preheat a grill or a large grill pan to high.
Grill meat for 10 to 15 minutes (about 7 1/2 minutes per side), or to desired doneness. (best a medium rare, about 145 degrees F). Set meat aside, cover and let rest for 5 to 10 minutes.
Slice the meat paper-thin with a long sharp slicing knife or a countertop deli slicer. Divide the meat among 8 bread slices, spread some Horseradish Cream on each one, then add a few rings of onion slices and top with remaining bread slices. Slice sandwiches in half. Makes 8 sandwiches (about 16 servings).