Add the egg, lemon juice, Dijon mustard, and salt to a tall jar or mixing bowl. Let sit for 1–2 minutes to settle.
Pour the oil gently over the top so it floats above the egg mixture.
Place the immersion blender at the bottom of the jar, touching the base.
Turn the blender on and keep it still until the mixture at the bottom turns thick and pale.
Slowly lift the blender upward as the mayonnaise emulsifies and becomes smooth and creamy.
Once fully combined and thick, taste and adjust salt or lemon juice as needed.
