A rich-but-worth-it appetizer, with prosciutto tucked under a blanket of bechamel sauce.
Snap off most tough outer leaves from artichokes. Trim 1 1/2 inches from top. In pot of water, place artichokes, 1 1/2 lemons, thyme, peppercorns, and bay leaf. Cover with smaller-size lid to keep artichokes submerged. Cook over medium heat until hearts are tender when pierced with knife, about 30 minutes.
Heat oven to 350 degrees. When artichokes are tender, drain, remove remaining leaves, and discard all but the smallest, most tender ones. Set those aside. Remove choke completely with melon baller or spoon, and rub artichoke bottoms with remaining lemon half. In a glass pie pan or gratin dish, strew leaves and mix in half of bechamel. Lightly butter artichoke bottoms, season with salt and pepper, and place on top of bechamel. Place folded piece of prosciutto in each bottom; fill with a rounded mound of bechamel.
Bake 20 to 25 minutes or until golden. Rub the tops with butter, and bake for 5 minutes more. Serve one or two bottoms per person along with some of the leaves.
The artichoke bottoms and the bechamel sauce can be prepared the day before you need them, then assembled and baked just before serving.