Slice the stems from the jalapeño peppers and place the peppers into a medium-sized pot. Add the chopped red onion to the pot as well. Pour in enough water to fully cover the vegetables. Bring the mixture to a boil over high heat, then reduce the heat and let it simmer for about 15 minutes, or until the jalapeños soften and turn dull green. You should be able to easily pierce them with a fork, but make sure they don't become mushy.

Strain the boiled jalapeños and onion out of the pot, making sure to reserve the cooking liquid. Set the strained vegetables aside for blending. I always save some of the boiling liquid to help adjust the sauce consistency later.

Transfer the boiled jalapeños and onion to a food processor. Add the garlic cloves, neutral oil, and salt. Blend the mixture until it is very smooth and creamy, which usually takes about 4–5 minutes. If you prefer a thinner sauce, gradually add a few tablespoons of the reserved boiling water until you reach your desired consistency. Finally, taste and adjust the salt if needed.

Pour the smooth jalapeño sauce into a serving bowl or jar. Adjust the seasoning to taste and serve as desired. The sauce can be enjoyed immediately or refrigerated for later use. For extra flavor, I like to let the sauce chill for a few hours to let the flavors blend together.