In a small food processor or cup using an immersion blend, whisk together yolks, lemon juice and zest, and mustard. Pulse, scraping down the sides, until it becomes a pale yellow paste.
While whisking or while the immersion blender is on, very slowly pour the oil into the yolk mixture. It should start to thicken and combine, but if it it does seperate, stop adding the oil and continue to whisk until it combines again, then resume the oil.
When fully combine, add the garlic and salt, either giving a few more quick pulses or folding in by hand. If you plan to add chives or black pepper, fold them in as well.
Cover and refrigerate for 1 hour to let flavors marry.
If you've tried this recipe, come back and let us know how it was in the comments or ratings.