Add the avocado oil, egg, lemon juice, Dijon mustard, and salt to a tall jar or container that is just slightly wider than the immersion blender head.
Place the immersion blender directly on the bottom of the jar and keep it still. Blend on high speed for about 30 seconds until the mixture at the bottom becomes thick and creamy.
Slowly lift the immersion blender upward while continuing to blend so the remaining oil fully emulsifies into the mayonnaise.
Transfer the mayonnaise to an airtight container and refrigerate until ready to use.
Notes
Whole Egg: I recommend using a pasteurized egg because raw eggs carry a tiny risk of salmonella, and since this mayonnaise is "raw," pasteurized eggs are a better option.
Avocado Oil: Any neutral tasting oil can be used. Avoid using extra virgin olive oil as it can make your mayo bitter.
Dijon Mustard: Regular yellow mustard, mustard powder and whole grain mayo can be used as substitutes.
Use a jar barely wider than the immersion blender head. This is the best option for your homemade mayonnaise to be successful.
It is important not to move the immersion blender from the bottom of the jar until the sauce starts to emulsify. Once it becomes thick on the bottom, slowly pull it up to incorporate the rest of the oil and ingredients.
Store leftovers in an airtight container or jar in the fridge for up to 3 days.