Tender flat iron, or shoulder top blade steak, stays juicy in the center when quickly broiled.
Heat broiler, with rack set 4 inches from heat. Line a rimmed baking sheet with aluminum foil; set aside. Soak eight 6-inch wooden skewers in water for at least 10 minutes.
Meanwhile, on a work surface, chop garlic and rosemary, and sprinkle with a little salt. Press blade of knife back and forth across mixture to make a paste. Place in a medium bowl, and stir in oil; season with 1 teaspoon salt and 1/4 teaspoon pepper.
Cut steaks into 24 1 1/2-inch chunks. Add to bowl; toss to coat. Thread 3 chunks of beef onto each skewer; place skewers onto prepared baking sheet.
Broil (without turning) 4 to 6 minutes for medium-rare. Meanwhile, in a small bowl, stir together sour cream and horseradish; season with salt and pepper. Serve skewers with horseradish sauce.
Before cooking, remove any gristle that might be running down the center of the meat.