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cook shrimp in a large pot of boiling salted water just until opaque, about 30 seconds.
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transfer to a paper towel–lined baking sheet and let drain.
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bring hot sauce, lime juice, soy sauce, and tajín seasoning to a gentle simmer in a large skillet over medium-low.
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whisking constantly, gradually add butter a few pieces at a time.
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whisk until all butter is incorporated and sauce is emulsified.
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add shrimp and cook, tossing gently and adjusting heat as needed to keep sauce just below a simmer, until cooked through, about 5 minutes.
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serve on top of onion on a platter.