Crack the egg into a tall jar. Add the Dijon mustard and sea salt and then place an immersion blender in so that it is touching the bottom of the jar.

While blending, slowly trickle the avocado oil into the jar.

Move the blender slowly up and down in the jar to incorporate the oil.

Keep blending until the mixture has become a thick, glossy mayonnaise. This takes several minutes.

Add the lemon juice and blend to mix.

Remove the blender and stir in the ketchup until fully mixed.

Put the lid on the jar and store in the fridge.