How to Make It
Preheat the oven to 350°F. Line a rimmed baking sheet with parchment paper.
Set a medium mixing bowl on a kitchen towel (this will prevent the bowl from moving as you whisk). Whisk together the egg yolk, garlic, and salt. Whisking constantly, slowly drizzle in the oil, drop by drop at first, until the mixture begins to thicken, then in a slow, steady stream until all the oil has been incorporated and the mixture is very thick. Whisk in the warm water to thin the aioli, then season to taste with additional salt, the lemon juice, and pepper. Serve right away or transfer to a lidded jar and refrigerate; the aioli will keep for up to 3 days in the refrigerator.