1. Crack egg into blending cup without breaking the yolk. Add lemon juice, mustard and salt. Then add avocado oil (it should go last!).
2. Insert the immersion blender all the way to the bottom of the cup (trapping the egg yolk within the blade), turn it on and don't move for 20 seconds (the suction creates a similar effect we get by slowly pouring in oil). Then slowly lift and continue blending until the mayo has thickened.
3. Store covered in the fridge for about a week.