Preheat oven to 425 degrees F. In a small bowl stir together garlic, marjoram, lemon peel, cracked black pepper, and the first 1/2 teaspoon salt. Brush tenderloin with 1 tablespoon of the olive oil. Sprinkle half the garlic-pepper mixture evenly over roast, patting to coat. Place meat on a rack in a shallow roasting pan. Insert a meat thermometer into the center of the meat. Roast tenderloin for 35 to 40 minutes for medium rare (140 degrees F) or 45 to 50 minutes for medium (155 degrees F).
Meanwhile, toss potatoes with the remaining 2 tablespoons of olive oil, the remaining garlic-pepper mixture, and 1/2 teaspoon salt. Add potato mixture to roasting pan during the last 30 minutes of roasting.
Remove tenderloin and potatoes to serving platter. Cover loosely with foil. Let stand 15 minutes (the meat's temperature will rise 5 degrees during standing). With a sharp knife carve into 1/2-inch slices. Garnish with marjoram and rosemary sprigs and lemon wedges, if desired. Makes 8 servings.