In your kitchen blender, place 1 cracked egg, 1/4 C avocado oil, 1 tablespoon of apple cider vinegar, 1 teaspoon prepared mustard (or 1/4 cup mustard powder) and 1/2 teaspoon of salt.
Pop the blender lid into place and whirl ingredients together for 7-10 seconds.
Leave the blender running and measure out the remaining 1/2 cup avocado oil.
Slowly drizzle avocado oil in through the hole in your blender's lid while the machine is still running, emulsifying oil with the other ingredients.
When nearly all the oil has been added, you'll notice that the mayo has thickened and is sticking to the sides of your blender, instead of running back down into the whirling blades.
Once all the oil has been added, whirl things together for another 10 seconds.
Remove the lid and use your spatula to scrape the thick, white mayo into a glass jar.
Taste test and add more salt, if desired.
Cover jar with a lid and refrigerate for 10-14 days.
Use in your recipes just like you would store bought mayonnaise.