Cut the cauliflower into small florets and peel and chop the onion. Cook the cauliflower and onion together in boiling, lightly salted water for about 12-15 minutes until tender. Drain thoroughly in a colander, then return to the saucepan and mash with a potato masher. This will release more moisture, so reheat gently to evaporate this, stirring so that it doesn't burn.
Meanwhile, grate the zest from the lemon. Add this and the (low fat) soft cheese to the potato-onion mix then stir in the nutmeg (optional). Season with salt and freshly ground black pepper, stir well, then serve garnished with thyme sprigs.