Usually made of mashed potatoes, garlic and olive oil, skordalia is a thick sauce served with meats, fish, fritters or greens.
Puree first 6 ingredients in blender or processor. Score large muscles of lamb to form even thickness. Rub yogurt mixture over lamb. Transfer to shallow dish. Cover and chill overnight.
Place beans in processor. With machine running, add garlic through feed tube and chop. Gradually add oil and puree, scraping down sides as needed. Season to taste with lemon juice, salt and pepper. Stir in parsley. (Can be made 4 hours ahead. Cover; let stand at room temperature.)
Prepare barbecue (medium-high heat) or preheat broiler. Season lamb with salt and pepper. Grill lamb about 10 minutes per side for medium-rare. Let stand 5 minutes. Cut lamb against grain into thin slices. Serve lamb with skordalia and bread.