Place a metal steamer basket in a large saucepan and pour in water to a depth of 2". Add 2 tablespoons lemon juice. Bring water to a boil, then add artichokes. Cover, reduce heat, and simmer artichokes until stems and hearts are easily pierced with a paring knife, 30–40 minutes. Meanwhile, blend Parmesan, yogurt, oil, lemon zest, remaining 1 tablespoon lemon juice, and 3 tablespoons water in a blender until smooth; season with salt and a generous amount of pepper. Serve artichokes with yogurt sauce for dipping.