Preheat oven to 425F (220C), place oven rack in the center of the oven. Line a sheet pan with silpat or parchment paper.
Remove the stems of the portobello and set aside (they don't need to be roasted like the tops.. they're already dry). Turn the mushroom caps, bottoms up, on a work surface. Using a small metal spoon, gently scrape out the "gills" on the underside of the mushroom. Doing this helps reduce the moisture content further (toss the gills into the yard, garden or compost!). Place the mushroom caps, bottoms up, on the sheet pan and roast for 20-25 minutes. This helps dry them dry out.
While the mushrooms are roasting, add the oil to a saute pan and heat until it shimmers. Turn the heat to low to medium-low and add the onion, stirring occasionally until tender, about 10 minutes.
Meanwhile, slice the stems removing the smallest amount possible at the base to remove the tough edge. Use two forks to "pull" the mushroom stems separating pieces with your fingers if needed until they are shredded. Set aside in a medium bowl.
Remove the portobello from the oven and once cool enough to handle, and using two forks again, shred the mushrooms, separating larger pieces with your fingers if needed until they are shredded. Add them to the same bowl with the shredded stems.
To the sauted onions, add all the shredded mushrooms, BBQ sauce and liquid smoke. Cook on medium-low for about five minutes to warm through stirring occasionally. Put the lid on to keep warm until ready to serve. Serve on warmed buns, stuffed into a sweet potato or cooled slightly in sturdy lettuce wraps. Top with coleslaw or pickles and serve with potato salad, corn on the cob, potato salad, baked beans... anything that you love with BBQ!
Store in a lidded container in the refrigerator for up to three days or in the freezer for up to two weeks. Thaw in the fridge overnight and gently reheat on the stove-top.