Preheat oven to 400° F (200° C). Grease a 9x13 baking dish.
Dry brush mushrooms and gently twist off stems. Using a small spoon, scrape out the brown gills. (*see photo above) Place them in the baking dish and lightly season with salt & pepper.
Add the hearts of palm to a food processor and give 3-4 long pulses, until shredded. Don't over process. The mixture should have a crab-like consistency. (*see photo above)
To a large bowl, add mayonnaise, lemon juice, mustard, worcestershire sauce, nutritional yeast, kelp, Old Bay Seasoning, parsley, garlic powder, salt & fresh cracked pepper. Combine well.
Add the shredded hearts of palm, bread crumbs, and green onion. Combine well. Taste for seasoning and add more if needed.
Using a small spoon, stuff each mushroom cap and place in the greased baking dish (leave a small amount of room in between each mushroom). Sprinkle the tops with panko breadcrumbs and drizzle with oil if desired.
Place in oven and bake for 20-30 minutes, until heated throughout and lightly browned on top. Serve hot. (sprinkle with fresh-cut parsley, crushed red pepper, or vegan parmesan cheese, if desired).