Thinly slice the red onion by cutting in half, peeling, then slicing as thin as possible. If you have a mandolin, use that.
Step 2
Add the onions to a large bowl along with the whole black peppercorns, salt and lime juice, vinegar and oregano if using. Mix it all together.
Step 3
Continuously mix the onions to make sure they're all getting coated in the lime juice. After 30 minutes they should be perfectly wilted and ready to enjoy.