-Both types of mushrooms can be found in Asian markets. They are tricky to replace because of their texture and flavor but other mushrooms can be used. If you can't have mushrooms, try replacing with some sliced hearts of palm.
-Use this recipe as a guide. If you like onion, add more! I love avocado so I always add in extra. Some other nice additions would be a splash of tomato juice/sauce, crushed nori or dulse seaweed, or a bit of fresh orange juice.