Fill medium pot with 10 cups water. Adjust amount of water as needed based on size of pot and size of jalapeños.
Place pot on stovetop over high heat. Heat pot until water begins to boil rapidly.
Once water begins to boil, add 6 large whole jalapeño peppers to pot. Immediately reduce heat under pot to low.
Simmer jalapeños until peppers are very tender, approximately 10 to 15 minutes.
When peppers are tender, carefully pour water and peppers into colander. Drain well, then transfer jalapeños to cutting board.
Carefully cut off stems of jalapeño peppers and discard. If desired, slice jalapeño peppers open and remove core and seeds.
Place peppers in blender. Let peppers cool slightly, then add 3 large cloves garlic, ½ cup avocado oil, 1 ½ teaspoons salt, 1 small white onion, and 3 tablespoons fresh cilantro leaves.
Secure lid on blender and blend ingredients until fully combined into smooth, creamy mixture. Pause blender and scrape down sides of bowl with silicone spatula if needed. Note: If salsa is too thick, blend in additional avocado oil in 1-tablespoon increments until desired consistency is achieved.
Once all ingredients are combined and mixture is completely smooth, transfer salsa to airtight container. Let salsa cool completely, then secure lid on container and place salsa in refrigerator.
Chill salsa 1 hour or until ready to serve. Serve chilled or at room temperature.