For the smoked pork: Preheat a smoker to 300 degrees F. using a 50/50 mixture of pre-soaked apple wood and oak woodchips.
Combine the granulated sugar, brown sugar, paprika, chili powder, garlic powder, onion powder, salt, pepper, cayenne pepper and dry mustard in a bowl and mix well.
Dry the pork and then rub the piece of meat with the spice rub, making sure to cover the entire piece. Place the pork butt directly on a rack in the smoker and cook until it reaches an internal temperature of 200 degrees F., 3 to 4 hours.
Remove the pork from the smoker and let it rest for 20 to 30 minutes, and then pull the meat, discarding large pieces of fat. Hold the pulled pork for later use.
For the BBQ sauce: While the pork is smoking, heat a pot over medium-high heat. Melt the butter and then add the onions and garlic and saute until the onions are tender. Add the chili powder, smoked paprika, dry mustard, garlic powder, onion powder and cayenne. Stir the spices into the butter and toast for about 1 minute.
Add the brown sugar and molasses and cook until the sugar is dissolved, 2 to 3 minutes.
Whisk in the ketchup, Dijon mustard, cider vinegar and Worcestershire sauce and bring to a simmer. Simmer the sauce for 30 minutes and then season with salt and pepper. Let cool slightly, and then puree in a blender.
For the potatoes: Preheat the oven to 400 degrees F. Toss the potatoes with the grapeseed oil, salt and pepper in a large bowl and then spread them out on a baking sheet. Bake until tender and golden brown, 20 to 25 minutes.
For the roasted portobello mushrooms: Remove the stems from the mushroom caps and scrape the gills out from the caps with a spoon. Starting at the outer edge of each mushroom cap, use a knife to peel the "skin," or outer layer, off the cap.
Arrange the mushrooms on a baking sheet and drizzle with the grapeseed oil. Season with the salt and pepper and roast the mushrooms stem-side up until the mushrooms are tender, about 10 minutes.
For plating: Heat a large saute pan over high heat. Heat the grapeseed oil and then add the bacon and onions and saute until cooked and tender. Season with salt and pepper and then add the pulled pork and BBQ sauce and cook until hot.
Divide the pork mixture and mound it on the 6 mushroom caps. Top each with a slice of cheese. Wipe out the saute pan and arrange the mushrooms inside. Add about 1/4 cup water, cover the pan, and cook to melt the cheese.
Divide the potatoes among 6 plates. Place the mushroom on top of the potatoes. Garnish with parsley and a pinch of freshly ground black pepper.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.