Preheat the oven to 325 degrees. Squeeze some of the juice from the lemon halves into a large bowl of water. Working with 1 artichoke at a time, snap off the dark green outer leaves. Slice off all but 1 inch of the remaining leaves. Peel and trim the bottom and stem of the artichoke. Halve the artichoke and scoop out the furry choke with a spoon. Cut the artichoke halves in half again, rub with the lemon, and add both the lemon and the artichoke to the lemon water. Repeat with the remaining artichokes.

In a large, ovenproof skillet, combine the 3 tablespoons of lemon juice with the olive oil, garlic, thyme, and bay leaf. Drain the artichokes and add them to the skillet. Bring to a simmer over moderate heat, stirring the artichokes occasionally.

Transfer the skillet to the oven and roast the artichokes for about 40 minutes, stirring a few times, until they are tender and browned in spots. Discard the bay leaf. Stir in the parsley and season the artichokes with salt and black pepper. Let cool.

Arrange the prosciutto slices on a platter. Spoon the artichokes and their cooking liquid into a bowl and serve with bread and the prosciutto.

The artichokes can be prepared through Step 3 and refrigerated overnight. Serve at room temperature.