Wasabi may be a mysterious looking substance, but it’s ingredients are quite simple. Wasabi is actually a plant. It grows leafy shoots (also edible) but the real power punch is in the root. The root is traditionally grated and formed before eating. That’s it. Maybe some water.
But, it should be noted, not all restaurants serving wasabi are actually serving the root based plant. It’s easier to substitue a mixture of horseradish, mustard, green food coloring and a binder (likely corn starch). To get the real stuff, you have to go to Japan or high end restaurants. If you are craving the authentic wasabi, you can always try Amazon for real wasabi, but make sure to read the ingredients. Otherwise, you are just buying expensive, green horseradish paste.