Add all the ingredients to a tall bowl or container. Using the immersion blender, mix everything until it has a thick, creamy consistency, about 20-30 seconds. Use as needed. Refrigerate well sealed up to a week.
200 ml grapeseed oil, or any unflavored vegetable oil, 1 egg, 1½ teaspoons Dijon mustard, 1 tablespoon white wine vinegar, apple cider vinegar, or lemon juice, 1/2 teaspoon salt