Preheat the oven to 200°C/fan 180°C/gas mark 6. Wrap the beetroot in foil. Place in a baking dish and roast for 1 hour 30 minutes, until tender. Allow to cool slightly.
Once the beetroot is cool enough to handle, roughly chop and put in a food processor along with the chickpeas, tahini, garlic, half the lemon juice and 3 tsp oil. Season well, then pulse until you reach your desired consistency – it should be smooth but still lightly textured, with no big chunks.
Heat a griddle pan until smoking and rub the sourdough slices all over with the oil. Toast on each side until golden.
To serve, spoon the dip into a sharing bowl and sprinkle with the lemon zest, sesame seeds and parsley. Drizzle over the remaining oil, then dip the toast in and enjoy!