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1
Bring a large pot of salted water to a boil.
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2
Have a large bowl of ice water close at hand.
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3
Immerse the spinach leaves briefly in the boiling water in batches to wilt and then transfer them swiftly to the ice water with a slotted spoon.
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4
As soon as the spinach is cool, use your hands to squeeze as much moisture out of it as possible.
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5
Chop the spinach and place it in a medium bowl.
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6
In a 10-inch skillet, melt 2 tablespoons of the butter with the olive oil over low heat.
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7
Add the leeks and saute until the leeks are wilted and very tender, about 15 minutes.
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8
Add 1/4 teaspoon salt, the mustard, 3/4 teaspoon white pepper and the apple butter, breaking up the apple butter as much as possible.
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9
Remove from the heat.
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10
Add the leek mixture to the spinach.
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11
To make the cream sauce, melt the remaining 2 tablespoons butter in a small saucepan over low heat.
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12
Add the flour and cook, whisking vigorously, for 1 minute.
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13
Gradually whisk in the milk and 2 large pinches each of salt and white pepper.
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14
Cook the mixture slowly over very low heat, stirring continuously and slowly with a heatproof rubber spatula, until thickened.
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15
Whisk in the cheese and continue to cook, stirring, until the sauce is smooth.
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16
Do not ever let the sauce boil.
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17
When the sauce is done, pour it over the spinach and leeks, stirring well to combine.
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18
Allow to cool for a couple of minutes, then taste for seasonings and serve warm.
