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1
Place tomatillos and serrano chiles in a small saucepan with just enough water to barely cover. Bring to a simmer and cook until tomatillos soften and turn dull olive, about 8 minutes. Do not drain.
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2
Transfer tomatillos, serranos, and all cooking water to a blender. Blend until smooth.
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3
Add the charred, peeled Anaheim and poblano chiles, salt, and half the cilantro. Pulse to a rough, uneven consistency — not a smooth purée.
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4
In a bowl, crush the avocado pulp by hand or with a fork to a lumpy texture. Fold in the blended chile mixture, crème fraîche, onion, and remaining cilantro.
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5
Taste and adjust salt. Serve at room temperature.