In a cast iron skillet, over a very high heat, char the onion slices and remove. Toast the garlic, remove and peel.
Add 2 tablespoons of the olive oil to the skillet and sear the chilies until charred. Add the remaining oil, the oregano, lime juice, and salt ingredients and cook 1 minute. Place all the ingredients in a food processor fitted with a metal blade. Process until they are roughly chopped but smooth.