Combine shallot, vinegar, and Dijon mustard and let macerate for at least 15 minutes. Slowly whisk oils into shallot mixture (or, if you have a jar with a lid, add oils, screw on lid, and shake to combine). Season to taste with salt and pepper. DO AHEAD: Cover and chill for up to 1 week in an airtight container. To freshen the dressing after a few days, add a squeeze of lime or lemon juice.
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