Place cauliflower florets in a food processor and briefly blitz until it resembles the texture of rice or couscous. You may have to do this in batches to avoid overcrowding the food processor. If you don't have a food processor, you can either finely chop the cauliflower or coarsely grate it.
Transfer to a large glass or microwavable bowl and microwave on high for a few minutes - this steams the cauliflower and evaporates moisture to make it extra fluffy. If you don't have a microwave, spread cauliflower out on an oven tray lined with baking paper and bake in a 200°C oven for 10-15 minutes until just tender and fluffy.
Toss hot cauliflower rice with butter/extra-virgin olive oil, herbs and lemon zest (if using), and season to taste with a little salt and pepper.