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1
Peel the kohlrabi, slice into quarter-inch-thick slices, then cut the slices into matchsticks a quarter-inch broad.
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2
Set aside.
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3
Make the sauce.
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4
Melt the butter in a small, heavy saucepan, stir in the flour and cook over very low heat, stirring often, for 15 minutes.
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5
Add the milk in a thin stream, beating with a wire whisk.
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6
Raise the heat to low-medium, stir in the half-and-half and cook the sauce, stirring almost constantly, until it smoothly coats a spoon, about 10 minutes.
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7
Keep warm over very low heat, stirring from time to time.
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8
Put the kohlrabi in a basket steamer over boiling water and cook for 8 to 10 minutes, or until just tender.
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9
It should be firm to the bite but not Chinese-crisp.
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10
When the kohlrabi is almost done, beat the egg yolk in a heatproof bowl, beat in a bit of the warm sauce and then add the yolk mixture to the saucepan.
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11
Cook, stirring, until the sauce is very thick and hot, but do not let it boil.
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12
Season the sauce with salt to taste and just enough nutmeg to give a toasty flavor without being too assertive.
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13
Remove the basket from the steamer and toss the contents gently until the surfaces are completely dry, then combine the kohlrabi with the sauce and serve at once.
