Preheat oven to 350°F (175°C).
Wipe the king oyster mushrooms down with a clean cloth to remove any grit. Chop off 1 inch (2½ cm) of the stalk to remove the tough base. Carefully but firmly drag the prongs of a fork down the length of each mushroom to shred the “meat” (this is easier if you lay the mushroom down). Place the shredded mushrooms in an ovenproof dish.
To make the rub, combine the paprika, onion powder, mustard powder, celery seeds, cinnamon, garlic powder, cayenne, sea salt and brown sugar in a bowl. Drizzle the mushrooms with the sunflower oil and sprinkle three heaping tablespoons (45 g) of the rub over the top. With clean hands, massage the oil and spice rub into the mushrooms before baking them in the oven for 25 minutes.
To prepare the po’boys, slice the baguette into three sections and slice each section in half lengthways. Smear a good dollop of mustard on the base half and lay a few slices of ripe tomato on top. Add a dollop of vegan mayonnaise to the top half of the baguette.
When the mushrooms are ready, remove them from the oven and transfer them to a large non-stick pan and stir with the BBQ sauce to warm for a minute.
To finish the po’boys, add a few good spoonfuls of the mushrooms mixture on top of the tomato followed by sliced pickles if using. Top with the other baguette half and serve.