Add egg yolk and minced garlic to a bowl. Whisk until well combined.

Add lemon juice and whisk until the mixture looks smooth and slightly thickened.

Begin adding oil very slowly, one drop at a time, while whisking constantly.

Once the mixture starts to thicken, continue adding oil in a thin steady stream while whisking.

Whisk until the aioli becomes thick, pale, and creamy.

If the sauce becomes too thick, add a few drops of water or lemon juice to adjust consistency.

Add salt to taste and stir in Dijon mustard if using.

Let the aioli rest for about 10 minutes before serving.