Ingredients

• 1  egg
• 1 tablespoon lemon juice (from 1/2 a lemon)
• 1 teaspoon Dijon mustard
• 1 cup avocado oil
• 1/4 teaspoon salt

Instructions

1. Place egg, lemon juice, and mustard in the bottom of cup or jar that just fits the head of your immersion blender. This is vital. the circumference of the jar must be just larger than the head of your blender and the egg/lemon juice mixture must reach the blades for this to work. If the mixture does not reach the blades, double the recipe before attempting.
2. Pour oil on top and allow to settle for 15 seconds. Place head of immersion blender at bottom of cup and turn it on high speed. Do not pulse or move the head. As mayonnaise forms, slowly tilt and lift the head of the immersion blender until all oil is emulsified. Season mayonnaise to taste with salt. Store in a sealed container in the refrigerator for up to two weeks.

From Serious Eats.