Melt the butter in a frying pan, and add the sliced leek and minced garlic. Cook over a medium-low heat for 5-10 minutes, until the leek is very soft but not overly browned.
Peel the sweet potato, and cut it into thin slices (ideally 2-3mm).
Transfer the cream into a jug, and add the smoked paprika, salt and pepper, and dried thyme. Mix thoroughly to combine.
Pour about a tablespoon of the cream mixture into the bottom of a baking dish (mine measured approximately 8 x 8 inches), and spread it around. This helps to stop the potato sticking to the bottom of the dish.
Now begin layering your ingredients. Add a single layer of sweet potato slices, scatter a few buttered leeks on top, and a light sprinkling of finely grated parmesan. Drizzle over a little more of the cream mixture. Repeat these layers until you've used all of the ingredients. Finish the dish by pouring any remaining cream on top, and scattering over any remaining cheese.
Add a lid to the dish, or cover tightly with some kitchen foil. Bake at 190°C (Gas Mark 5 / 375°F) until the sweet potato is soft, and offers no resistance when you insert a sharp knife. My bake took about 1 hour 25 minutes, but the exact time will depend on exactly how thinly you sliced your potatoes.