Place ¼ cup of oil in a food processor, and add the egg, ground mustard, and salt.
Process thoroughly, for about 30 seconds, until combined and pale yellow in color.
With the blender running, slowly drizzle in the remaining oil until the mixture is emulsified and thick. Adding the oil slowly makes for a creamier mayonnaise. The total pouring time to aim for should be 60-90 seconds.
With the food processor still running, add the lemon juice, and mix briefly, for only about 5-10 seconds, until incorporated.
The mayonnaise is now ready to be used.
Store in the refrigerator for up to 4 days.