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1
Make the kale filling: in a large saute pan, melt the butter over medium heat. Add the leeks and saute until tender, 2-3 minutes. Add the garlic and saute until fragrant, 1 minute.
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2
Add the kale to the pan and cook, stirring occasionally, until completely wilted and very tender, 12-15 minutes. Season with salt and pepper. Stir in the sage. Transfer the mixture to a large bowl to cool completely.
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3
Season with salt and pepper. Stir in the herbs and cook for 1 minute more. Remove the mixture from the pan and cool completely.
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4
Next, prepare the squash: place the squash on a baking sheet and rub all over with olive oil. Season with salt and pepper and roast until the squash is just barely fork tender in the thickest park, 25-30 minutes. Cool completely, then rub all over with the mustard.
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5
Preheat the oven to 425u00b0 F, and line a baking sheet with parchment paper.
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6
On a lightly floured surface, roll out each piece the puff pastry to 1/4 inch thick; both pieces should be slightly longer and wider than the squash.
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7
Transfer one piece of pastry to the prepared baking sheet; this will be the base of the Wellington. Arrange the kale mixture in an even layer in the center of the pastry, leaving 1 inch of excess all the way around. Use a paring knife to trim away any and discard any excess dough.
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8
Place the squash on top of the kale mixture, centering it evenly. Egg wash the edge of the pastry, and gently drape the second piece of dough over the squash. Use your fingers to gently press the dough at the edges together to seal (once again, use a paring knife to trim any excess from the top piece of dough).
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9
Use a fork with lightly floured tines to crimp all the way around the Wellington; press firmly to make sure the puff pastry pieces are sealed together.
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10
If desired, you can score the top of the pastry with a paring knife to create a decorative pattern. For a sharper look, egg wash the pastry before you score it-the score marks will be paler than the surface of the pastry and the pattern will be very clear! For a more even look, score the pastry after scoring it-the pattern will be more subtle and the pastry will be evenly golden brown. Use the tip of a sharp paring knife to barely score the surface of the dough without cutting fully through it; create any design you like!
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11
Transfer to the oven and bake until the pastry is golden brown and fully baked through, and the squash is very tender when pierced, 35-40 minutes. Let rest for 10-15 minutes before slicing and serving.
