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1
Heat a large pot of water over high heat, while you prepare the bechamel. If it boils before the sauce is finished, turn down the heat slightly so it will return quickly to a boil later.
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Combine onion slices, diced carrots, diced celery, thyme, peppercorns, and milk in a small saucepan and heat over medium low. Once warm, gradually raise heat until milk just boils, then remove from heat and set aside, covered.
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3
In a medium saucepan, melt 4 tablespoons butter over medium heat, then add flour and stir or whisk constantly for a few minutes as it bubbles slightly. Reduce heat to medium low and carefully whisk while pouring in the milk mixture with all its additions. Once combined, raise heat to medium high and bring to a simmer. Stir, simmering, for a couple minutes, until slightly thickened. Reduce heat to low and stir frequently, ensuring that no flour accumulates in the corners of the pan, for about 15 minutes.
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4
Set a mesh strainer over the pan used to heat the milk, and pour sauce through it, stirring to encourage all the sauce through the strainer (which may take a few minutes; see notes for alternatives to straining). Stir in cream, then season with 1/2 to 3/4 teaspoon salt, 1/4 teaspoon pepper, and a pinch of freshly grated nutmeg. Set aside sauce.
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5
Add 1/2 tablespoon salt to the now-boiling water. Lower rutabagas into the water, maintain high heat, and blanch for 2 minutes (water may not return to a boil in that time). Drain rutabaga and rinse briefly with cold water.
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Preheat oven to 375 F. Melt 1 tablespoon butter in a medium skillet over medium heat. Add diced onion and cook until softened, but barely browned, about 8 minutes. Place onion in a large bowl and add blanched rutabaga, turnips, julienned carrot, 1 teaspoon salt and 1/4 teaspoon pepper (or more seasoning to taste). Toss to combine.
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7
Lightly grease a deep, 8 by 10 inch (or thereabouts) casserole pan or baking dish with butter or oil.
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8
Distribute the vegetable mixture in an even layer. Pour the bechamel sauce evenly over the vegetables, spreading gently with the back of a spoon to cover, if needed. Top with a layer of all the breadcrumbs, then drizzle with a tablespoon of olive oil.
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9
Bake on middle oven rack for 45 minutes, until bubbling and top is slightly golden. Remove from oven. Cool for 5 to 10 minutes before serving. Leftovers keep well in the refrigerator for a few days. See notes for make ahead instructions.
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Notes:
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1. As an entree, the dish serves about 4.
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2. If you don't have time to strain the bechamel, either use edible, finely chopped aromatics (onion, carrot, celery, garlic) or flavor the milk with a bay leaf and bouquet garni of peppercorns and thyme, removing both just before adding milk to the roux.
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3. To substitute ground nutmeg, start with the smallest pinch possible, tasting and adding more if needed.
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4. To assemble up to 12 hours in advance, leave breadcrumbs and oil off until ready to bake and refrigerate in baking dish, covered. Bring to room temperature for an hour before baking, to keep approximately the same cook time. If baking straight from the fridge, add 10 or more minutes, and make sure the gratin is heated through before removing from oven and serving.
