Add the egg, mustard, lemon juice or vinegar, and salt to the bottom of a mason jar. Slowly pour in oil. Allow to sit for a minute until the oil is floating on top.

Place the stick blender directly over the egg and tight to the bottom of the jar. Turn on and hold in place for about 30 seconds, or until most of the oil is incorporated.

Slowly lift, bit by bit, to blend in any remaining oil. If necessary, work the blender up and down in the jar a time or two. Do not overblend.

Keep for up to five days in the coldest part of the fridge; if using pasteurized eggs, keep up to two weeks.