Slice onions into thin rings. Add garlic, salt and pepper to vinegar.
Add oysters. Let stand for 2 to 3 hours.
If you are planning to serve it 5 hours later or more.
Boil the kilawin over high heat for a few seconds and remove immediately or it will be reduced to the size of a pea.
You could serve it with the vinegar mixture or drain the oysters and serve.