Peel and crush (mince) the garlic. Put the cream, garlic and thyme sprigs (or rosemary if you prefer) into a small saucepan and slowly bring to just below boiling (never allow cream to boil). Remove from the heat, season with salt and freshly ground pepper to taste and leave to infuse.
Place the frozen spinach into a colander, pour over a kettle of boiling water and leave to drain for a few minutes. Squeeze out as much water as possible, season with salt, freshly ground pepper and freshly grated nutmeg.
Peel and thinly slice the sweet potatoes - about ⅛" (3mm) thick. Use the butter to grease an ovenproof dish and spread half the potato slices on the bottom. Add a layer of spinach, then the remaining potato slices. Pour over the cream mixture, through a sieve to remove the garlic and herbs, then sprinkle with grated Parmesan or Cheddar (or other hard cheese of your choice). Bake in a preheated moderately hot oven (Mk 6 - 400ºF - 200ºC) for 45-55 minutes until golden and tender.
If you are not a vegetarian, serve hot with Lamb Chops.