Add the avocado, lemon, mustard (if using) and salt to a food processor or high speed blender and process till it's blended up well. Stop a few times to scrape the bowl down if need be.
With the motor running, drizzle in your olive oil. Continue blending till the mixture has taken on a creamy, light texture.
Makes 1/2 to 3/4 cup (depending on the size of your avocado).