2. Skewer the onion slices using two parallel skewers to hold all the layers together. Make sure one skewer pierces the center section so that it doesn't fall out. Rub all slices with olive oil and salt to taste. Set aside.
3. Trim the ends off of 1/2 a bunch of cilantro, then add cilantro to the food processor along with the vinegar, salt, and pepper.
4. Blend on high until the sauce is bright green and resembles a pesto or chimichurri style sauce.