Preheat the oven to 375 degrees.
Using a food processor, pulse the cream cheese until smooth and creamy, about 1 minute. Add the vegan mayonnaise, sour cream, drained hearts of palm, ground mustard, lemon juice, Old Bay seasoning, coconut aminos, nori, and hot sauce. Pulse until the hearts of palm are chopped and combined. About 30 pulses.
Using clean hands, carefully twist the stems off of the button mushrooms. We want to make sure not to tear or rip the mushrooms as we will be filling the center of the mushrooms.
When all the stems are removed, using a spoon, spoon one teaspoon worth of filling into each mushroom. You don't want to overfill the mushrooms or the filling will boil over in the oven.
When all the mushrooms are filled, place the mushrooms on a baking sheet and bake for 20 minutes.