In a small bowl, whisk together the mustard, vinegar and salt. Gradually whisk in the olive oil until emulsified, then whisk in the buttermilk. Season to taste with pepper and onion juice-- add more salt, if needed.
This sounds wonderful. I've been making my own butter at home for a while now, and am always looking for new ways to use the buttermilk that is left over from the process.
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