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1
To make the aillade, unshell the pistachios and cook them over medium heat just long enough to warm through, 2 to 3 minutes. With a mortar and pestle (or sharp knife or mini food processor), pound the garlic with 1/2 teaspoon salt until a paste forms. Add the pistachios and pound into small, irregularly shaped pieces. Stir in the lemon zest and olive oil. Taste and adjust the seasoning.
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2
Bring a large pot of generously salted water to a boil. Add the pasta and cook until al dente.
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3
Meanwhile, in a large skillet, heat olive oil and butter over medium heat. Add spring onion, asparagus pieces, piment d'espelette, and a few large pinches of kosher salt, stirring to evenly coat in the fat. Saute until crisp-tender, stirring often, about 4 to 6 minutes. Add lemon juice, starting with less and adding more, to taste. Adjust salt to taste.
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4
When pasta is done, drain (reserving about 1/2 cup of cooking water) and toss with the asparagus in the skillet (or in the pasta pot if the skillet isn't large enough). Stir in about half of the pistachio aillade, and then add more, as needed, until the pasta is evenly coated. Add a few tablespoons of cooking water, as needed, to thin the sauce. Adjust acidity and seasoning to taste. Off the heat, add feta, tarragon, and parsley. Serve warm, at room temperature, or cold.
