Combine soy milk, vinegar and Cajun seasoning. Toss the mushrooms to coat and marinade for 2 hours, re-tossing once or twice.
Combine the all-purpose flour, baking powder and garlic powder. Dredge each mushroom slice in the flour, shaking off the excess, and fry in hot oil in a skillet until golden, turning once. Keep enough oil in the skillet so that it is always about ¼ inch deep. Drain fried mushrooms on a paper towel lined plate.
Meanwhile mix together the mayo, lemon juice, Worcestershire, Dijon, hot sauce, and garlic. Spread the mixture over the buns on both the top and bottom. Top with romaine, tomato slices and the fried mushrooms.
Recipe by Alison's Allspice at https://www.alisonsallspice.com/fried-portabella-poboy-sandwiches/