Place the garlic, egg yolk, lemon juice, mustard and water into the cup of an immersion blender. Pour the oil on top of the other ingredients and wait a few moments for the oil to separate from the ingredients beneath it. Gently nestle the blade of the immersion blender into the egg yolk at the bottom of the cup. Turn on the blender and blend until the mixture is fully emulsified, about 1 minute. Add salt to taste.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.