Preheat the oven to 220 °C/Fan 200 °C/400 °F/gas 6.
Toss cauliflower florets in oil and spread across a large baking tray and roast for ~30 minutes. Check regularly throughout this time and turn over pieces as needed to roast evenly.Because the browning makes so much of this soup’s flavour, it’s important to take the time it needs and don’t progress to the next step until it's ready.
In a large saucepan, gently fry the onion until it turns golden, then add the garlic and cook briefly - just until the raw smell is gone. The moment it's just cooked enough immediately add the stock to stop the frying and lock in the garlic's bright, fresh flavour.
Add the cauliflower (reserve a few florets for garnishing later), bring it to a boil and then turn down to simmer for 15-20 minutes until the cauliflower is soft.
Blend until smooth. To season, taste it and then blend in the lemon juice and nutmeg. Add salt and pepper, and a little more lemon juice if needed until the flavours come out.
Serve with the reserved roasted cauliflower florets and chives to garnish.
Keeps well covered in the fridge for up to three days.