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1
Char tomatillos and serrano chillies directly over gas flame or under broiler until blackened and blistered, 4–5 minutes; transfer to blender.
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2
Add quartered onion, cilantro leaves, salt, and cumin seeds to blender.
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3
Pulse until texture reaches coarse purée with visible specks of chilli skin and herb; do not overblend.
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4
Transfer to serving bowl; stir in lime juice and adjust salt to balance acidity and heat.