From Diana Kennedy's book "The Essential Cuisines of Mexico."
2 Medium white onions, finely sliced into rings, about 3 cups
4 cups boiling water
1/2 cup seville orange juice
1/2 cup of water
1 scant tablespoon dried mexican oregano
1 Habanero chile, finely sliced
salt to taste
Immerse the onion rings in a sauce pan of boiling water for a few seconds until slightly blanched. Drain well and put into a nonreactive bowl. Mix in the rest of the ingredients, and set aside to season for at least 2 hours before using.