Feel free to buy buttermilk. I can only ever find it in 1L containers and never end up using. This method is made to order. White vinegar can also used in place of lemon juice.
To make your own buttermilk, combine milk and 1/2 of the lemon juice and let stand for 5 minutes.
In a food processor, pulse remainder of lemon juice, green onions, garlic, dill, basil, mustard, olive oil, salt and pepper until finely minced. Add buttermilk, yogurt and mayonnaise and process until smooth.
Refrigerate for at least one hour, preferably overnight to allow the flavours meld.