Combine the vinegar and milk together and allow it to sit and "curdle" into homemade buttermilk. Whisk all the ingredients together until smooth. Chill for at least 30 minutes - 1 hour. It will thicken more the longer it chills. Serve on a simple salad or use as a dip for veggies.
Sometimes the onion flavor can get a little more strong on day two. If you don't like that flavor, just reduce or omit the shallots.
