Combine the almond milk and ground flax in a blender. Blend on high speed until the flax meal is barely noticeable and the mixture becomes frothy.
Add the sugar, dry mustard, onion powder, salt, vinegar and lemon juice, and blend again for a few seconds.
Slowly add the oil... add one fourth a cup at a time, blending between each addition. The mayonnaise should thicken the more oil you add. If it looks watery, just keep blending.
Transfer to glass container and refrigerate for a few hours (this will make it even thicker).
Recipe by The Veggie Girl at https://www.theveggiegirl.com/2014/05/08/vegan-soy-free-mayo/