Peel and slice the red onions into very thin rings or half-rings. Place them in a clean glass jar or heatproof bowl.

Slice the habanero thinly. Wear gloves if you’re sensitive; avoid touching your face. Add the slices to the jar with the onions.

In a small saucepan, combine the vinegar, water, salt, and sugar. Heat over medium until the mixture reaches a boil and the salt and sugar dissolve.

Remove the pan from heat and let the brine cool for 1–2 minutes. Stir in the lime juice.

Carefully pour the warm brine over the onions and habanero, ensuring everything is submerged. Sprinkle in a pinch of dried oregano if using.

Stir gently to distribute the brine, then press the onions down so they’re mostly covered. Seal the jar with a lid.

Let cool to room temperature, then refrigerate. You can use the pickles after 15–30 minutes, but they’re best after a few hours.