Crack 1 egg into a wide-mouth jar, carefully keeping the yolk intact (this is crucial!). Add ½ a juiced lemon, ½ tsp of mustard, and ¼ tsp salt. Finally, pour in 1 cup of oil and let it settle for about a minute.
Place your immersion blender directly over the egg yolk and press it to the bottom of the jar. Turn the blender on high and hold it steady.
This part is key, slowly move the blender up to ensure all the ingredients are fully emulsified. It should take about 1 minute to get to the top of the jar.
Transfer the mayonnaise to an airtight container, or use it immediately in your favorite recipe. Homemade mayo will keep in the fridge for about one week.