Position an oven rack 4 inches from broiler element and heat broiler to high. In small bowl, make vinaigrette by combining lemon juice, sour cream, garlic and pinch of salt. Slowly whisk in 1/4 cup olive oil.
In medium bowl, combine lamb with 1 tablespoon olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper. Toss to coat evenly. Thread lamb onto 4 small (8-inch) bamboo or metal skewers.
Put skewers on broiler pan and broil lamb, flipping once, until browned on outside but still pink inside (medium doneness), 2 to 4 minutes per side. Transfer skewers to small, shallow baking dish. Whisk vinaigrette to recombine and pour 3 tablespoons of it over skewers, turning to coat.
In medium bowl, toss arugula and onion with enough of remaining vinaigrette to lightly coat (you may not need all of it). Season with salt and pepper to taste. Pile greens on 2 plates, top each salad with 2 lamb skewers and sprinkle with cheese.