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1
Combine beets, thyme branches, garlic and peppercorns in a large pot.
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2
Cover with cold salted water.
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3
Bring to a simmer over medium-high heat; cook until beets are tender, 15 to 30 minutes depending on size.
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4
Remove with a slotted spoon.
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5
Add greens and cook for 2 minutes (do this in batches if necessary); remove with tongs and transfer immediately to a bowl of ice water.
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6
Drain well.
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7
Once beets are cool enough to handle, peel them and slice crosswise into 1/4-inch-thick slices.
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8
Roughly chop greens leaves and stalks.
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9
Prepare the bechamel: In a small saucepan over low heat, melt 5 tablespoons butter.
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10
Stir in 75 grams flour ( 2/3 cup).
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11
Cook, stirring constantly, for 2 minutes; roux should smell cooked but remain white.
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12
Slowly whisk in milk until mixture forms a thick, smooth sauce.
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13
Stir in 8 ounces Cheddar (2 cups) until melted.
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14
Stir in mustard powder, Worcestershire and Tabasco.
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15
Season with salt to taste.
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16
Make the crumble topping: In a small bowl, stir together remaining 2/3 cup flour, the oats and the hazelnuts.
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17
Use your fingers to work in 6 tablespoons butter, the remaining 1 ounce Cheddar ( 1/4 cup) and the Parmigiano-Reggiano.
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18
It should be a mixture of large and small pieces.
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19
Season with a pinch of salt, pepper and the nutmeg.
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20
When you are ready to assemble the dish, heat oven to 350 degrees.
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21
Butter a 2-quart gratin or baking dish.
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22
Spread a layer of bechamel on the bottom.
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23
Top with a layer of beets, followed by a layer of greens and stalks.
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24
Season generously with salt and pepper, and sprinkle with thyme leaves.
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25
Spread another layer of bechamel on top and repeat process to fill dish (you will end up with 3 or 4 layers).
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26
Cover entire surface with crumble topping.
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27
Transfer dish to oven and bake until bubbling and golden brown in spots, 45 minutes to 1 hour.
