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1
Preheat the oven to 400.
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2
On a rimmed baking sheet, season the chicken legs with salt and pepper and drizzle with the olive oil.
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3
Roast the chicken for about 30 minutes, until browned and cooked through.
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4
Let stand until cool enough to handle.
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5
Shred the chicken, discarding the skin and bones.
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6
Reserve the pan drippings.
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7
Reduce the oven temperature to 375.
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8
In a large saucepan, melt the butter.
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9
Add the onion and leek and cook over moderate heat until tender, about 5 minutes.
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10
Stir in the mustard and the flour and cook until pasty, about 1 minute.
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11
Whisk in the chicken broth, 3/4 cup of the milk, the heavy cream and the chicken pan drippings and bring to a simmer.
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12
Cook over moderate heat, stirring frequently, until thick, about 5 minutes.
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13
Stir in the shredded chicken and the tarragon and season with salt and pepper.
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14
Let stand until cool, about 30 minutes.
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15
Spread the chicken filling in an 8-by-11 1/2-inch baking dish.
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16
On a lightly floured work surface, roll out the puff pastry to a 9-by-13-inch rectangle.
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17
Lay the pastry over the baking dish and gently tuck in the overhanging pastry.
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18
Using a sharp knife, cut 3 slits in the top of the pastry.
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19
In a small bowl, beat the egg with the remaining 1 tablespoon of milk.
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20
Brush the egg wash over the pastry and sprinkle with the sesame seeds.
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21
Set the pie on a rimmed baking sheet and bake for about 1 hour, until the puff pastry is golden brown and the filling is bubbly.
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22
Let stand for 30 minutes before serving.
