Crack egg directly into jar. Add lemon juice and salt, as well as mustard, garlic and black pepper if using.
Pour oil directly over egg and friends and let sit 30 seconds for ingredients to settle.
Insert your immersion blender, making sure it makes contact with the bottom of the jar.
Blend on high for 20 seconds WITHOUT moving the blender, until emulsion happens and mayonnaise climbs all the way to the surface.
Slowly lift blender to surface while blending 10-15 seconds more.
Move blender around edges to incorporate any remaining oil.
Store covered in refrigerator up to 10 days.
Notes
Must use light-tasting olive oil, NOT extra virgin.
Jar must be narrow enough for immersion blender to fit snugly
Large or extra-large eggs work best
Mayo will thicken from bottom up during blending. Do not move blender until emulsion is almost complete.
Use the optional add-ins for even better results: Dijon mustard acts as additional emulsifier making mayo more stable, while garlic contains compounds that aid emulsification