Combine the spices for the meat in a resealable 1-gallon plastic bag and shake to mix thoroughly. Add 1 piece of meat, shake it around in the bag, remove it and repeat with the second piece of meat. Add the first piece back to the bag, seal and marinate as long as you can in the refrigerator -- 24 to 48 hours is recommended. But if you can't wait, just go for it. It won't be as over-the-top, but it will still be really good.
Make the sour cream-horseradish sauce: 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 on grilling, remove the meat from the refrigerator and let it sit at room temperature. Preheat a grill or a large grill pan to high.
Grill the meat for 10 to 15 minutes (7 1/2 minutes per side), or until desired doneness. (It's best at medium rare, about 140 degrees F or 145 degrees F.) Set the meat aside, cover and let rest for 5 to 10 minutes.
Slice the meat paper-thin with a knife or a countertop deli slicer. Divide the meat among 8 bread slices, spread some sour cream-horseradish sauce on each one, then add a few rings of onion slices and top with the rest of the bread.
Photograph by Yunhee Kim
Recipe courtesy Guy Fieri for Food Network Magazine