Combine Ingredients: Use a tall jar (a wide mouth mason jar would work or a tall cup that's wide enough for an immersion blender). Add egg to the jar. Then add oil, mustard, lemon juice and salt.

Blend: Place immersion blender at the bottom, over the egg yolk. Blend without moving for 10–15 seconds until thick, then slowly lift blender stick up and down until everything is mixed smoothly. Taste and add additional mustard, salt or lemon juice, if needed. If it's too thick, add 1 tsp of water. If it's too thin, add 1 tsp of oil.

Combine Ingredients: Add all ingredients to the blender except the oil. Slowly drizzle oil in while blender is running, until thick.