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1
Heat the oven to 400 degrees.
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2
Cut the dried base and green leaves from the cauliflower and discard them.
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3
Separate the head into florets about the size of walnuts and chop the stem into similar size pieces.
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4
Cut the potatoes into similar-size pieces as well.
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5
Bring a large pot of water to a boil over high head and salt liberally.
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6
Add 1 tablespoon of vinegar and the cauliflower and potatoes (the vinegar will help keep the cauliflower white).
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7
Cook until the cauliflower pieces are tender enough to be easily cut with a spoon, 10 to 15 minutes.
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8
Drain and set aside.
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9
While the cauliflower is cooking, make a cheese sauce.
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10
In a medium, heavy-bottom saucepan, melt the butter over medium-low heat and stir in the leeks.
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11
Cook until they are soft, about 10 minutes.
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12
Add the flour and whisk to make a smooth paste.
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13
Add the milk a little at a time, cooking until it thickens.
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14
When all the milk has been added, reduce the heat and cook over medium-low heat for 5 to 10 minutes.
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15
Stir in the creme fraiche and then one-half cup of the Gruyere, 1 teaspoon salt and a generous grating of nutmeg (a little less than one-quarter teaspoon).
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16
Whisk until smooth, then taste and add more salt or nutmeg if necessary.
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17
Butter a 6-cup gratin dish and spread a thin layer of the sauce evenly over the bottom.
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18
Arrange the cooked cauliflower and potatoes in an even layer over the sauce.
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19
Pour the remaining sauce over the top and spread evenly with the back of a spoon.
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20
It should come about three-quarters of the way up the vegetables.
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21
Scatter bread crumbs evenly over top and then scatter the remaining one-quarter cup Gruyere over that.
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22
Bake until the gratin is bubbling and the top is browned, 30 to 40 minutes.
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23
Serve immediately.
