Slice the leeks into ¼-inch rounds or strips about 2 inches long, and wash them well. Quarter the fennel bulb, trim away most of the core, and slice it lengthwise into pieces about ¼ inch thick. Peel the potatoes or turnips, and slice them into rounds 1/8 inch thick. Cut away the gnarly surface of the celery root, cut it into quarters, and slice it thinly. If it is not to be used right away, cover it with water that has been acidulated with a few spoonfuls of lemon juice or vinegar.

Preheat the oven to 375°F. Butter the bottom and sides of a baking dish, and rub with the crushed garlic clove. Layer half of the vegetables in the dish, and season them with salt, freshly ground white pepper, and the lemon thyme or thyme leaves. Make a second layer with the rest of the vegetables, and season them as well. Add the cream, or milk and cream, and dot the surface with small pieces of butter, using about 2 tablespoons in all. Lay a piece of foil loosely over the top and bake.

Melt the rest of the butter, and toss it with the bread crumbs. Remove the gratin from the oven after ½ hour, take off the foil, and cover the surface with the bread crumbs. Return the gratin to the oven and continue baking until the vegetables are tender, about another ½ hour. Remove the gratin from the oven, and let it rest 5 to 10 minutes before serving.