1. Fill a large bowl with water and squeeze the lemon halves into it; add the lemon to the bowl. Cut off all but 1 1/2 inches from the artichoke stems. Working with 1 artichoke at a time, pull off all the green outer leaves, leaving a cone of pale yellow leaves. With a sharp knife, cut off all but 1 1/2 inches of the leaves. Peel the stem and trim any tough green skin from the artichoke bottom with a sturdy vegetable peeler. Using a melon baller or small spoon, scoop out the hairy choke. Drop the trimmed artichoke heart into the bowl of water. Repeat with the remaining artichokes.
2. Bring a large saucepan of water to a boil. Cut each artichoke heart through the stem into 8 wedges. Add the wedges to the boiling water and cook until crisp-tender, about 8 minutes. Drain and rinse under cold water; pat dry.
3. In the same saucepan, combine the stock with the cream, thyme sprig and 1 teaspoon of salt and bring to a boil. Add the potatoes and cook over moderately low heat until barely tender, about 8 minutes. Fold in the artichokes and season with salt and pepper.
4. Butter a 9-by-13-inch baking dish or 3-quart gratin dish. Pour the contents of the saucepan into the dish; discard the thyme sprig. Sprinkle the Gruyère over the gratin and cover with foil.
5. Preheat the oven to 375°. Bake the gratin for 30 minutes. In a small bowl, toss the bread crumbs with the butter. Remove the foil, sprinkle the crumbs over the gratin and bake for about 30 minutes longer, or until most of the liquid has been absorbed and the vegetables are tender.
6. Preheat the broiler. Broil the gratin about 6 inches from the heat for about 2 minutes, or until the crumbs are crisp and golden; turn the dish as necessary for even browning. Serve hot.
Make Ahead: The unbaked gratin can be refrigerated overnight.