Preheat the oven to 200°C/180°C fan/gas 6 and grease a 24 x 16cm ovenproof dish with butter.

In a large saucepan over a medium heat, heat the cream, whole garlic, thyme sprigs and a good grating of nutmeg to just below boiling point. Remove from the heat and set aside to infuse.

Melt the butter in a frying pan over a medium heat, add the onion, spring onions and crushed garlic and sauté for 2 minutes, until softened but not coloured. Add the spinach, season with salt and pepper, and sauté for a further 3–4 minutes, stirring well to combine. Remove from the heat.

Layer half of the sweet potato slices over the bottom of the oven dish. Pour over the spinach mixture, then sandwich tightly with another layer of sweet potato. Strain the infused cream through a fine sieve and pour over the dish. Sprinkle with the grated cheese, chopped thyme leaves and allspice, cover with greaseproof paper and bake in the oven for about 30 minutes.

Remove the greaseproof paper and continue to bake for a further 10–15 minutes, or until the sweet potatoes are tender and the top is golden.