Add the soy milk, mustard, and salt to a tall container.
Stick your immersion blender in and turn it on at a low speed for a couple of seconds.
Slowly drizzle in the oil as you blend. Go slow and steady—this is key! Once the mixture starts to thicken, it helps to move the immersion blender up and down gently.
When it’s totally thick, like a paste (upside-down test: it shouldn’t run out of the container 😉), and looks like mayo, stir in the lemon juice.
Give it one last quick blend. Done!Keeps in the fridge for about a week in a sealed jar. (But let's be honest—it’ll be gone way before that.)