Measure and add lemon juice, vinegar, mustard and sea salt.
Pour oil over the top of the previous ingredients.
Place stick blender into the jar and cover the egg yolk with the blender. Turn the blender on and watch for the mixture to begin turning white and thickening. Move the blender around just a little to make sure all ingredients are combined. This should take around 1 minute of blending.
Can be used right away. Store in the refrigerator for up to 10 days.