Step 1

Crack egg into the base of a jar that's mouth is wide enough to fit your immersion blender, or alternatively into the cup attachment for your blender, making sure the yolk doesn't break.

Step 2

Add dijon, a pinch of salt and lemon juice.  Gently pour in the cup of oil, taking care not to break the yolk.  Insert the immersion blender so it's touching the bottom of the jar, making sure the blades completely cover the yolk.

Step 3

Blend for 20 seconds without moving, until the majority of the jars contents now resemble mayonnaise -  white and thick.  There will be some oil not incorporated at the top and that's ok.  Move the stick blender up and down to incorporate it.  Taste, then season with more lemon juice or salt if required.  Push the blender back through the mayo to ensure all is incorporated.

Step 4

Serve immediately or store in a sealed glass jar in the fridge.