Stir lemon juice into 3 cups cold water in a medium bowl. Wash artichokes with tap water. Bend back the leaves of each artichoke, snapping them off, until you reach the core of pale inner leaves. Cut off the sharp tips of the remaining leaves and slice off the stem near the base; reserve stem. Quickly place the artichoke bottoms into the lemon water to help prevent discoloration. Remove one artichoke bottom from lemon water and trim the sides and base with a sharp knife, removing the tough dark green layer. Using a teaspoon, scoop out and discard the hairy choke. Return artichokes to lemon water while preparing the filling.
For filling, trim away the dark green layer from each reserved stem. Dip stems in lemon water, then coarsely chop. Place the chopped artichoke stems, prosciutto, garlic, mustard, pepper, and olive oil in a small bowl. Drain artichoke bottoms. Spoon about 1 to 2 Tbsp of filling into each artichoke bottom, mounding the filling slightly, if necessary.
Bring 1/4 cup water to boiling in medium skillet. Carefully add the stuffed artichoke bottoms. Reduce heat. Simmer, covered, for 20 to 25 minutes or until the artichoke bottoms can be pierced easily with a fork.
Transfer artichokes to a baking sheet. Sprinkle with Parmesan cheese. Broil 5 to 6 inches from the heat for 1 to 2 minutes or until the cheese is melted and lightly browned. Serve warm.Servings: 2