Add all the ingredients (except the salt and pepper) to a blender and blend until combined and smooth.
Taste, then season with salt, pepper and extra lemon juice, if desired.
Notes
For best results use room-temperature soy milk and blend 30-60 seconds until it thickens. Keeps about 5 days refrigerated. For a gluten-free version, check that your Dijon mustard is certified gluten-free.