1 (8- to 10-pound) bone-in leg of lamb, fat trimmed to 1/4-inch thick
8 medium garlic cloves, peeled
¼ cup packed fresh flat-leaf parsley leaves
2 tablespoons fresh rosemary leaves
2 tablespoons fresh oregano leaves
2 tablespoons plus 1/4 teaspoon kosher salt, divided
1 ½ tablespoons olive oil, divided
1 ¼ teaspoons black pepper, divided
2 pounds fingerling potatoes (about 6 cups), halved lengthwise
1 ½ pounds leeks (about 3 large leeks), white and light green parts only, sliced into rounds (about 6 cups)
Pat lamb dry. Using the tip of a small, sharp knife, score lamb by making shallow cuts all over. Process garlic, parsley, rosemary, oregano, 2 tablespoons of the salt, 1 tablespoon of the oil, and 1 teaspoon of the pepper in a food processor until smooth, about 30 seconds. Rub mixture all over lamb. Place lamb on a rimmed baking sheet lined with parchment paper; chill, uncovered, at least 8 hours or up to overnight.
Preheat oven to 450°F with rack in lowest position. Remove lamb from refrigerator; let stand at room temperature 30 minutes. Roast lamb in preheated oven for 1 hour, 20 minutes. Remove from oven.
Toss together potatoes, leeks, and remaining 1/4 teaspoon salt, 1/2 tablespoon oil, and 1/4 teaspoon pepper in a bowl. Spread in a single layer around and under lamb on baking sheet. Reduce oven temperature to 350°F, and return baking sheet to oven. Roast until a thermometer inserted in thickest portion of lamb registers 125°F to 130°F for medium rare, and potatoes and leeks are tender, about 45 minutes. Remove from oven; let rest 30 minutes. Slice lamb. Arrange lamb and potato-leek mixture on a serving platter, and serve.