1For the quick pickle, combine the vinegar and sugar plus 1 tsp salt in a bowl. Add beetroot and toss to coat. Set aside for 15 minutes.
2To make the black garlic aioli, mash black garlic with a fork then combine with aioli. Preheat chargrill over high heat.
3Brush the steak with olive oil then season well with salt and freshly ground black pepper. Cook for 4 minutes a side for medium rare then rest, lightly covered in foil for 5 minutes.
4Toast the sourdough on the chargrill until slightly blackened. Top a slice of sourdough with tomato, drained beetroot pickle, avocado, cheese, sliced steak, aioli, rocket and another slice of bread. Repeat for other sandwiches.