With a sharp knife, slice about 1 inch off the top of each artichoke. If leaves have thorns, snip off tips of leaves with kitchen shears. Cut stems, leaving about 1 inch on each artichoke. Rub cut parts of artichokes with lemon to prevent browning.Fill a large pot with 2 to 3 inches of water (water should only come up to the bottom of a steamer basket, not cover it) and place on high heat. Insert steamer basket into pot and add artichokes to basket; cover. Bring to a boil, then reduce heat to a simmer. Cook for 25 to 35 minutes or until outer leaves of artichokes can be easily pulled off.Meanwhile, add remaining ingredients to a medium bowl and whisk to combine. Refrigerate mixture until ready to serve. Serve steamed artichokes warm with lemon-herb sauce for dipping.