1. Whisk together the egg yolk and water in a mixing bowl.
2. Use a spoon to drip a few drops of olive oil into the egg yolk and whisk vigorously. Add a few more drops and whisk again. Do this until you’ve used about ¼ cup of the olive oil and have a thick, yellow, emulsified sauce in the mixing bowl. Pour larger volumes of olive oil in, still whisking, until olive oil is gone.
3. Whisk in lemon juice, salt, garlic and mustard (if using). Adjust lemon and salt to taste. The fresh garlic will mellow over a few days.
Store: In fridge for 2 weeks