Step 1

Combine the yolks, lemon juice, salt, and mustard: Combine the yolks, lemon juice, salt, and mustard in the blender cup or canning jar. Pulse with the immersion blender a few times to break up the yolks. You may need to tilt the cup so the blender blade reaches the yolks.

Step 2

Add 1/2 cup of the oil a little at a time: With the immersion blender running, add the first 1/2 cup of oil a few tablespoons at a time. Make sure each addition of oil is completely blended before adding the next. The mixture should start to thicken and lighten. (Once you've made this a few times and have a feel for it, you can try going more quickly, or even try pouring all the oil on top of the eggs and then blending all at once — going slowly at first is just an extra level of insurance.)

Step 3

Add the remaining oil in a steady stream: Once the first half cup of oil has been added, you can add the rest more quickly. Add as much of the oil as needed to reach the consistency you prefer; the more oil you add, the thicker the mayo will become. You may not need to use all the oil. If the mayo becomes too thick and you'd like to thin it out, blend water, 1 teaspoon at a time, into the mayo until you reach your desired thickness.

Step 4

Store the mayonnaise: Transfer any mayo not being used immediately to a storage container (or leave it in the canning jar and seal it with a lid). Homemade mayo will keep for about 1 week in the refrigerator.