How to Make It
Combine egg yolks, 1 tablespoon cold water, mustard, lemon juice, and salt in a blender. Process until frothy, about 20 seconds. While blender is running, slowly drizzle in vegetable oil through lid opening, followed by olive oil; process until smooth and thick, about 1 minute. Store in an airtight container in the refrigerator for up to 1 week.