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1
Slice the leeks into 1/4-inch rounds or strips about 2 inches long, and wash them well.
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2
Quarter the fennel bulb, trim away most of the core, and slice it lengthwise into pieces about 1/4 inch thick.
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3
Peel the potatoes or turnips, and slice them into rounds 1/8 inch thick.
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4
Cut away the gnarly surface of the celery root, cut it into quarters, and slice it thinly.
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5
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.
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6
Preheat the oven to 375F.
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7
Butter the bottom and sides of a baking dish, and rub with the crushed garlic clove.
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8
Layer half of the vegetables in the dish, and season them with salt, freshly ground white pepper, and the lemon thyme or thyme leaves.
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9
Make a second layer with the rest of the vegetables, and season them as well.
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10
Add the cream, or milk and cream, and dot the surface with small pieces of butter, using about 2 tablespoons in all.
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11
Lay a piece of foil loosely over the top and bake.
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12
Melt the rest of the butter, and toss it with the bread crumbs.
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13
Remove the gratin from the oven after 1/2 hour, take off the foil, and cover the surface with the bread crumbs.
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14
Return the gratin to the oven and continue baking until the vegetables are tender, about another 1/2 hour.
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15
Remove the gratin from the oven, and let it rest 5 to 10 minutes before serving.
