First, make sure all your ingredients are at room temperature. Grind garlic, lemon zest and salt in the mortar bowl with the pestle. Take care to grind in one direction only. Grind in fresh or dried herbs or spices at this time. A food processor will work, but some elbow grease makes anything taste better. It’s a zen thing.
Place the paste into a medium sized bowl. If you are adding something creamy, like Dijon mustard, now is the time to whisk it in followed by the egg yolks. Slo-o-o-wly add half of the oil while whisking to emulsify the sauce. Add the water and lemon while continuing to stir or whisk, then slowly add the remaining oil. Continue to stir until the mix thickens to a consistency just short of mayonnaise.
If the sauce gets too thick, mix in warm water a teaspoon at a time. If the emulsion separates, whisk it into a single egg yolk at room temperature to re-emulsify.
The aïoli can also be flavored up by introducing small quantities of desired herbs or seasonings into the original paste—like the aforementioned mustard. In a pinch, go with store bought—but homemade aioli is so much better. Both require refrigeration.