Preheat your oven to 190°C/gas mark 5 and line a baking tray with baking paper.
Pulse the cauliflower in a food processor until the texture is fine and grainy. Place in a microwaveable bowl and cover with clingfilm. Pierce a few times with a fork and cook on high for 1 minute, then stir. Cook again for 1 minute, stir, then allow to stand for 1 minute to cool slightly. Transfer the cauliflower to a muslin or clean dishcloth and squeeze any excess water out. Be careful, as the liquid will still be hot.
Transfer the cauliflower to a large mixing bowl. Add the eggs, cumin and half the coriander and mix well. Season and mix briefly again.
Make thin 17cm small circles with the cauliflower mix on the lined baking tray, making sure there are no gaps or holes in each one. Bake in the oven for 10 minutes until they start browning on the edges. Carefully flip them over and place back in the oven for 3-4 minutes. Place on a wire rack and leave to cool slightly.
Meanwhile, toss together the black beans, tomato, avocado, red onion and chilli with the lime juice and remaining coriander. Season to taste and serve wrapped in the cauliflower tortillas with a dollop of sour cream.