Whisk the mayonnaise and sour cream in a medium bowl until silky and smooth.
Slowly whisk in the buttermilk until the dressing is thick but pourable.
Stir in lemon juice, parsley, dill, chives, garlic powder, onion powder, salt, pepper, and optional MSG.
Taste and adjust salt, pepper, or lemon. Thin with a splash more buttermilk if needed.
Cover and chill for at least 30 minutes to let the flavors meld. Stir before serving.