Slice off the very top off the artichoke, then remove the tough outer leaves near the stem. Using kitchen shears, slice off the sharp tops of the leaves. Peel the stem, then cut off most of it, leaving only about a 2.5 centimeters. Remove the innermost pale leaves tipped with purple and discard. Cut artichokes into quarters. Place artichokes in a large bowl with water and lemon juice to prevent discoloration while you prepare the lamb.