A very basic version taught by my grandma:
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Slice 1 large red onion in half moon shape
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Place in a glass container and add enough lime juice to cover it. If lime is unavailable, lemon or white wine vinegar also work.
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Add a pinch of salt and oregano.
No heat is needed. Let it sit for at least an hour on room temperature or in the fridge if the weather is too hot.
Gotta love how unambiguous TLA are.