Add the jalapeños and chopped onion to a large pot.
Cover with water and bring to a boil. Reduce the heat and simmer for about 7-10 minutes, or until the jalapeño peppers are just softened but still bright green. They should be easily pierced with a fork but not mushy.
Strain the vegetables, reserving about 1/2 cup of the boiling water.
After boiling, remove the stems from the jalapeño peppers.
Add the cooked jalapeños and onion to a blender or food processor. Add the garlic cloves, apple cider vinegar, lime juice, salt, and avocado oil. Blend until the mixture is smooth and creamy, about 1-2 minutes.
If the sauce is too thick, add a few tablespoons of the reserved boiling water to thin it out to your desired consistency.
Taste and adjust, adding more salt or lime juice if needed.
Transfer the sauce to a container and refrigerate for up to 1 week. This sauce is not suitable for canning.