Step 1

Place the mustard, buttermilk, garlic, shallot, lemon zest, lemon juice, vinegar, peppercorns, and salt in a blender. Blend on low speed just so the ingredients get acquainted, about 45 seconds. They don’t have to be completely puréed—that’ll come later.

Step 2

With the blender running, open the top. Increase the speed to medium and slowly drizzle in the two oils, turning the speed up a little more (to medium-high) as you go.

Step 3

Stop the blender, remove the jar from the base, and taste for seasoning. (Do not dip your finger or a spoon in while the jar is on the base. Anything can happen—an electrical surge, a switch getting bumped. It’s not worth the risk. Safety first!) If it needs more salt (highly likely), add a couple more pinches.

Step 4

Add the parsley and dill, put the lid back on, and return the jar to the base. Starting on low speed and slowly increasing to high, purée for about 15 seconds to partially break down the parsley and dill.

Step 5

Serve immediately or refrigerate, covered, for up to 5 days. The flavors meld really nicely after a few hours in the fridge.