Heat a large, dry skillet over a medium heat and add the sliced mushrooms. Let the water come out of the mushrooms and once they start to shrink and brown, add in 1 tbsp of olive oil and stir. Cut the tofu into pieces a similar size to the cooked mushrooms and add to the skillet too along with ¼ cup of barbeque sauce, marmite and black pepper. Cook, stirring often, until the mixture has started to catch and burn slightly in places. Remove from the heat and stir in the remaining tablespoon of barbeque sauce and olive oil.
Heat the apple cider vinegar, salt and sugar in a small pot until steaming. Place the finely sliced red onion in a bowl or jar and pour the hot vinegar over. Let it sit for at least 30 minutes to soften and turn pink.
Cut the rolls in half. Spread on some vegan mayonnaise and mustard. Top with a few leaves of baby spinach if you’d like then spoon on the warm mushrooms and top with a few pickled onion bits. Sandwich together and serve warm.