Add egg yolks, garlic, and avocado oil to a 2-cup glass measuring cup (or other container that is just large enough to allow the head of your blender to reach the bottom); allow oil to settle. Place the head of an immersion blender on the bottom of the pitcher; turn it on to its highest speed. When you see aioli beginning to form, slowly lift the blender head from the bottom and move it up and down and side to side until all of the oil is incorporated. (recipe note #1)
For easier whisking, transfer to medium bowl and add olive oil in a slow, steady stream, whisking constantly until emulsified. Whisk in salt and lemon juice and zest. (recipe note #2) Taste for seasoning and add salt as desired; serve or refrigerate. Store refrigerated in a sealed container for up to 4 days.
If for some reason your aioli breaks (i.e., doesn't immediately turn thick), don't throw it away! It's an easy fix. Add an egg yolk to a fresh bowl, whisk it and then, very slowly, whisk in the broken mixture until it starts to emulsify. Continue to drizzle and whisk until you have a bowl of smooth, creamy aioli.
You may want to transfer the aioli to a separate bowl before whisking in the olive oil, salt and lemon juice and zest.