Preheat the oven to 180°C/fan 160°C/gas mark 5. Place the celeriac on a medium baking tray with a lip and drizzle with the oil. Rub seasoning all over, then cover in foil. Bake for 20 minutes.
Meanwhile, mix together all the barbecue sauce ingredients. Remove the celeriac from the oven and pour over half the sauce. Cover again with the foil and place back in the oven for 1 hour.
Remove from the oven and gently pull apart with two forks. Ladle over a little more of the sauce and cover again. Bake for a further 30 minutes. Remove from the oven, pull again, ladle over a little more sauce and bake for a final 30 minutes. Pull again and gently stir together, scrapping in the dark sticky edges. Leave to cool slightly before serving.
Meanwhile, add the avocado, lime juice, white wine vinegar and 4 tbsp water to a mini food processor. Blitz until smooth. Add the coriander and season well, then briefly blitz again. Add the cabbage and carrots to a medium bowl and pour over the avo dressing. Toss together until completely coated.
When ready to serve, layer the coleslaw and celeriac between the bread rolls.