For the "pulled pork": Preheat the oven to 300 degrees F, line two baking sheets with parchment paper and line a third baking sheet or pan with paper towels. Brush the parchment-lined baking sheets with about 1 tablespoon of vegetable oil each. Place the mushroom slices in a single layer, very close together, on the parchment and brush lightly with the remaining vegetable oil.
Bake the mushrooms until slightly softened and brown, 25 to 30 minutes, then flip the mushrooms over, rotate the pans in the oven, and continue to bake until the mushrooms are well-browned and the edges are crispy, 30 to 35 minutes. Remove from the oven and transfer to the paper-towel lined baking sheet to dry for a few minutes. (Leave the oven on.)
In a large bowl, whisk together the soy sauce, molasses, brown sugar, paprika and garlic powder. Carefully toss the baked mushrooms in the marinade until evenly coated. (You may need to do this in several batches.) Return the marinated mushrooms to the baking sheets in a single layer. Sprinkle with salt and a few grinds of pepper and return to the oven until crisp, about 5 minutes.
For the coleslaw: Meanwhile, combine the cabbage and carrots in a large bowl and set aside. Combine the mayonnaise, vinegar, celery seed and sugar in a small bowl and mix well. Add and salt and pepper to taste. Pour the dressing over the cabbage and carrots and toss to coat. Add more sugar, salt and pepper if desired. Chill until ready to serve.
To serve: Separate the party buns into halves. Place some of the mushrooms on the bottom half of the bun, then top with coleslaw. Add toppings as desired, then top with the top bun.
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