Tandoori Leg of Lamb

Name: Tandoori Leg of Lamb
Ingredients: 1/2 cup plus 2 tablespoons fresh lemon juice; Kosher salt; One 6-pound bone-in leg of lamb; 2 cups plain whole-milk yogurt; 1/2 cup vegetable oil; 2 garlic cloves, minced; 2 tablespoons minced peeled fresh ginger; 2 tablespoons dry mustard; 2 teaspoons ground cumin; 1 teaspoon ground nutmeg; 1 teaspoon ground cardamom; 1 teaspoon freshly ground black pepper; 1 teaspoon ground cinnamon; 1 teaspoon turmeric; 1 teaspoon cayenne pepper; 2 medium sweet onions, thinly sliced; 1/2 cup chopped cilantro; 4 tablespoons unsalted butter, melted; Lemon wedges, for serving