In a bowl or using an immersion blender, whisk the egg yolks, mustard, and lemon juice until smooth.
Slowly drizzle in the oil while whisking constantly until thick and emulsified.
Season with salt.
Whisk mayo, sour cream, and buttermilk until smooth.
Stir in seasonings and whisk to combine.
Adjust consistency with buttermilk and flavor with lemon juice.
Chill for 30 minutes before serving.
