Preheat the oven: Preheat your oven to 180°C (160°C fan) or Gas Mark 4. Lightly grease a baking dish with a small amount of low calorie cooking spray.
Prepare the sweet potatoes: Peel the sweet potatoes and slice them thinly, about 3-4mm thick. You can use a mandoline for even slices or a sharp knife.
Cook the onions and garlic: In a small pan, heat some low calorie cooking spray over medium heat. Add the sliced onion and cook for 5 minutes until softened. Add the minced garlic and cook for another 1-2 minutes. Remove from heat.
Make the sauce: In a mixing bowl, combine the low-fat crème fraîche, semi-skimmed milk, mustard powder mix, and half of the grated cheddar cheese. Stir in the thyme, and season with salt and black pepper to taste.
Assemble the gratin: Layer half of the sweet potato slices in the prepared baking dish, overlapping them slightly. Spread the cooked onion and garlic mixture over the top. Pour half of the sauce over the onions. Then, layer the remaining sweet potato slices on top and pour over the rest of the sauce.
Top with cheese: Sprinkle the remaining grated cheddar cheese over the top layer of sweet potatoes.
Bake: Cover the dish with foil and bake in the preheated oven for 30 minutes. Then, remove the foil and bake for an additional 15 minutes, or until the sweet potatoes are tender and the top is golden and bubbling.
Serve: Let the gratin cool for a few minutes before serving. Garnish with a sprinkle of fresh thyme if desired, and enjoy!