Place the oil, vinegar, mustard, and salt in a jar slightly bigger than the head of the immersion blender. You can use the jar or pitcher that came with the blender. Crack the egg into a bowl and carefully slide it into the jar without breaking the yolk.
Step 2
Insert the immersion blender into the jar and place the head of the blender over the yolk. Blend on low/medium speed and keep the blender still until you see white streaks forming.
Step 3
Keep the blender still until about half the mixture turns white. Increase the speed to medium/high and slowly tilt the blender to the sides to emulsify more oil. Finally, lift the blender slowly and blend as the oil transforms into mayo.
Step 4
Turn off the blender and remove it from the jar. Give it a quick mix to ensure that no oil remains. Taste the mayo and season with more salt if desired.
Step 5
Transfer it to an airtight container or a jar with a lid. Keep it in the fridge for up to 10 days. You can use it as a spread or a base for dressings and sauces.