Add 1 egg to the bottom of the mason jar. Next, add 1 tablespoon of vinegar, 1/2 teaspoon of mustard, and 1/2 teaspoon of salt. Then, pour 1 cup of vegetable oil (or oil of choice) on top of the other ingredients.
Allow the ingredients to sit in the jar for a few minutes. Giving time for the egg to settle at the bottom and the oil to disperse back to the top is key before blending.
Insert the immersion blender into the jar until it reaches the bottom. Begin blending the egg until it appears white and creamy. Then begin incorporating the other ingredients by slowly lifting the immersion blender towards the top to blend in the oil.
The blending process should take 3-5 minutes. The mayo should be able to hold stiff peaks while maintaining a jiggly appeal. Keep the mayo in the mason jar and store in the fridge for one week.