Add soy milk, apple cider vinegar, dijon mustard, and salt to a tall jar. The best jar to use is a tall, narrow container that fits the immersion blender, but doesn't have much excess room on the sides. If your immersion blender came with a blending jar, use that. I use an extra sourdough starter jar I have because it is slightly more narrow than my standard mason jars.
Very slowly pour the oil overtop. Allow this mixture to settle for a minute or two before blending. This allows the heavier ingredients to settle towards the bottom. This helps the immersion blender start by emulsifying the heavier ingredients first and gradually incorporate the oil.
Place the immersion blender on the bottom of the jar. Stick the immersion blender in the jar and press it against the bottom of the jar. Begin to blend, keeping the immersion blender on the bottom of the jar for the first 15 seconds of blending.
Slowly blend upwards. After 15 seconds, slowly begin to pull the immersion blender towards the top of the ingredients, blending as you go. Once you reach the top of the mayonnaise ingredients, everything should be fully emulsified.**If for whatever reason your mayonnaise is not thickening, do not throw it out! It can be saved with my troubleshooting step in the notes section below!
Transfer to storage jar. You can store the homemade mayonnaise directly in the jar you blend it in, but I find it easier to transfer it to a more shallow container.
Enjoy! Enjoy immediately or refrigerate until chilled.
Notes
NOTE 1: I think this mayonnaise should work with any neutral tasting oil. I have made this mayonnaise with success with the following oils:
Marianne's Harvest Avocado Oil
Chose Foods Avocado Oil
Great Value Vegetable Oil(this one can be finicky - use the below troubleshooting as needed) - I prefer this one, though, because it is significantly cheaper.
TROUBLESHOOTING: If your mayonnaise is not thickening for whatever reason, it's likely the oil was added too quickly. But don't throw it away! It can be saved! Get another jar that is similarly tall and narrow. Add 1/4 cup room temperature soy milk in the bottom. Place the immersion blender in it and blend it for about 10-15 seconds. After 15 seconds, keep blending, but very, very slowly add the runny mayo mixture. Keep blending until thick and creamy!
This recipe makes 1.5 cups of mayonnaise, or 12 ounces. A serving is 2 tablespoons.
Leftovers will keep in an airtight glass jar for up to 10 days.