1-1/4 cup of light olive oil, divided
1/2 -1 teaspoon mustard powder
1/2 teaspoon salt
1/2 to 1 lemon, juiced (about 2 teaspoons)
Place all ingredients in the jar or bowl – if your immersion blender came with a jar, that's perfect. The size of the jar or bowl is important. All of the mixture must reach the blades. Let ingredients settle for a few seconds.
Place head of immersion blender down into the mixture, at the bottom of the jar. Turn on high speed. Do not move the head until a white, creamy, thick mayo forms at the bottom by the head – this will take a few seconds. Begin to slowly tilt and lift the immersion blender in order to emulsify all the oil; turn the blender off and use the head to push down the oil at the top if necessary. When all oil is emulsified, remove blender and season with salt to taste.
Voila – Mayo!