Combine sour cream, yogurt, lemon juice and all ingredients except buttermilk and feta in a pint jar or quart sized bowl and mix well.
Add the buttermilk slowly, stirring until desired thickness is reached (see note to thin for a dressing). Fold in the feta. If using a bowl, transfer to a jar with a lid.
Refrigerate one or more hours before using, and taste to adjust seasonings again.
Shake well before each use. This tends to get thinner as it sits in the fridge- I just add a bit more sour cream, salt & pepper if I'd like it thicker.
Notes
*If using regular yogurt you may need to add less buttermilk to get the consistency you desire.To Thin for Dressing: add more milk 1 tablespoon at a time until a pourable consistency is reached.