Wash the mushrooms, pop stems off, then pull everything into little bits and add it to a pot.
Place the pot on the stove top, cover the mushrooms with salt and add in seaweed. Add in water then set the heat high, bring them to a boil and let them simmer down until the water is gone.
In a bowl add chopped green onion, minced Italian parsley, minced celery, minced garlic, lemon juice, horseradish, dijon mustard, white wine vinegar, hot sauce, vegan mayo, cayenne pepper, and cajun seasoning. Stir this, cover it up, and let sit in the fridge.
Add olive oil to mushrooms and stir. Allow them to cook until they are done steaming.
Once they are done, take sauce out of fridge and dump the mushrooms in the bowl. Mix it together well, cover it up, then put it back in the fridge (just until it's all nice and cold).
Butter up a hot pan, toast your buns on each side, and fill them with the mushroom salad! Top with some green onion and hot sauce and enjoy!