1. place the egg in the bottom of a wide-mouth jar. add the mustard, salt, lemon juice, and the olive oil. 2. Insert an immersion blender all the way to the bottom of the jar. start the blending but do not lift the blender until the mixture has thickened and turned white at the bottom of the jar. only then, slowly move the blender up, waiting for the oil to incorporate as you go, until you get the texture of mayonnaise. 3. Enjoy straight away, and refrigerate leftovers in a tightly sealed jar for up to 1 month. the mayonnaise will thicken once cooled in the fridge.