How to Make It
Squeeze the juice from 1 lemon half into a large bowl of water. Snap off the dark green outer leaves of 1 of the artichokes. Cut 1 inch off the top, then peel the bottom and stem. Halve the artichoke lengthwise and scoop out the furry choke. Cut each half in half lengthwise, then rub the quarters with the remaining lemon half and add them to the lemon water. Repeat the process with the remaining 8 artichokes.
In a large enameled cast-iron casserole, heat the oil. Add the onions, garlic and 2 teaspoons of salt and cook over moderate heat, stirring, until softened and light golden, about 15 minutes. Scatter the sunchokes over the onions in an even layer. Drain the artichokes and arrange on top. Pour in the wine and 2 cups of water. Completely cover the vegetables with the prosciutto.
Cover the casserole and cook over low heat for 30 minutes. Stir in the cream and sprinkle the scallions on top. Cover and cook until all of the vegetables are tender and the soup has thickened slightly, about 30 minutes longer. Remove from the heat and let stand for 10 minutes. Season with salt and serve.