Remove and discard the stems from the jalapeño peppers. Place the whole peppers in a medium saucepan, then add the chopped red onion.
Pour enough water into the saucepan to completely submerge the jalapeños and onion. Bring the water to a boil over high heat.
Once boiling, lower the heat and simmer the vegetables for approximately 15 minutes. The jalapeños should soften and change to a muted green color. They are ready when easily pierced with a fork but should not be allowed to become mushy.
Drain the cooked jalapeños and onion, reserving a portion of the cooking water. Set the strained vegetables aside.
Transfer the softened jalapeños and onion to a food processor. Add the garlic cloves, neutral oil, and salt.
Process the ingredients for 4 to 5 minutes, or until the sauce becomes exceptionally smooth, creamy, and evenly blended.
For a thinner consistency, incorporate the reserved cooking liquid a few tablespoons at a time, blending after each addition until the desired texture is reached.
Taste the finished sauce and add more salt only if necessary.
Transfer the creamy jalapeño sauce to a serving bowl or storage jar. Serve immediately or refrigerate until needed.