To make the sauce, place all the ingredients in a blender and pulse until smooth. Serve immediately or chill until ready to serve.
1 avocado, 1/2 cup mayonnaise, 1/2 cup sour cream, 1/2 cup roughly chopped cilantro, 1 chopped green onion, 3 Tbsp buttermilk, 1/2 Tbsp olive oil, 1/2 Tbsp lime juice, 1/4 tsp salt
Store in an airtight container in the refrigerator and use within one week.
Notes
Don't have buttermilk? Make your own by mixing 1 cup milk with 1 tbsp lemon juice and let it sit for 5-10 minutes to sour. Freeze the leftover buttermilk in an ice-cube tray for later.