1. Prepare grill.
2. Rub surface of lamb with 1 tablespoon lemon juice and olive oil. Sprinkle evenly with Greek seasoning and 1/2 teaspoon salt. Make several small slits on outside of lamb; stuff with garlic slices.
3. Coat lamb with cooking spray. Place on grill rack; grill 10 to 11 minutes on each side or until a thermometer registers 140°. Let stand 10 minutes. Drizzle remaining 1 tablespoon lemon juice over lamb. Chill lamb at least 1 hour.
4. While lamb chills, process mint, yogurt, sour cream, pepper, and remaining 1/4 teaspoon salt in a food processor until smooth, stopping to scrape down sides. Transfer mint sauce to a small bowl; stir in peas. Cover and chill until ready to serve. Thinly slice lamb; serve with mint sauce. Garnish with mint sprigs, if desired.
Note: To roast in the oven instead of grilling, place lamb on a rack coated with cooking spray in a roasting pan. Bake in a preheated oven at 425° for 32 minutes.