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1
In a medium saucepan, melt the butter.
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2
Add the shallot and cook over moderate heat until softened and lightly browned, 3 to 5 minutes.
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3
Stir in the flour and cook until bubbling, about 1 minute.
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4
Gradually whisk in the milk and bring to a boil, whisking.
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5
Cook over moderate heat, whisking frequently, until the sauce is thickened and no floury taste remains, 5 to 7 minutes.
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6
Stir in the mustard and nutmeg.
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7
Season the bechamel with salt and pepper.
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8
Let cool.
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9
Line a large rimmed baking sheet with parchment paper.
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10
On a lightly floured work surface, roll out each piece of puff pastry to a 10-by-6-inch rectangle.
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11
Slide 1 pastry rectangle onto the prepared baking sheet.
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12
Spread one-third of the cooled bechamel on the first pastry, leaving a 1-inch border all around.
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13
Sprinkle half of the cheese on top and cover with half of the ham.
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14
Repeat the layering with another third of the bechamel and the remaining cheese and ham.
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15
End with a final layer of bechamel.
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16
Cover the tart with the remaining pastry and press all around the edge to seal.
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17
Crimp the edge decoratively.
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18
Using a paring knife, cut four 1-inch slits in the top of the tart, then brush all over with the egg wash. Refrigerate until chilled, about 20 minutes.
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19
Preheat the oven to 450.
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20
Bake the tart for 20 to 25 minutes, until puffed and golden brown.
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21
Let cool for 10 minutes before serving.
