Remove the egg from the refrigerator 30 minutes before prep to bring it to room temperature. Alternatively, submerge the whole egg in a bowl of warm tap water for 5 minutes.

Add the room-temperature egg, lemon juice, Dijon mustard, and fine sea salt to the bottom of a clean, dry blender pitcher.

Secure the blender lid, removing the center lid cap. Blend the egg mixture on medium speed for 15 seconds until pale yellow, frothy, and completely uniform.

With the blender running on medium speed, begin pouring the avocado oil into the center hole in a pencil-thin, continuous stream. Take a full 90 to 120 seconds to pour the entire cup of oil.

Listen for the motor pitch to deepen as the mixture thickens into an opaque, glossy cream. Once all oil is incorporated, turn off the blender immediately.

Scrape down the sides of the blender with a silicone spatula. Taste and add up to 1 extra teaspoon of lemon juice or a pinch of salt if desired, pulsing once to combine.

Transfer the finished mayonnaise into a clean glass jar, seal tightly, and chill in the refrigerator for at least 30 minutes before serving.