Preheat the oven: Preheat your oven to 180°C (fan 160°C) / Gas Mark 4.
Prepare the potatoes: Thinly slice the potatoes using a sharp knife or mandoline for even slices.
Cook the onion and garlic: Heat a frying pan with low-calorie cooking spray. Sauté the onion and garlic on medium heat for 3-4 minutes until softened but not browned.
Create the sauce: In a bowl, mix the skimmed milk and vegetable stock together. Add the thyme, nutmeg, salt, and black pepper.
Assemble the gratin: Lightly grease a medium ovenproof dish with low-calorie cooking spray. Layer half of the potatoes evenly, followed by the sautéed onion and garlic. Pour half of the sauce mixture over the top.
Repeat: Add the remaining potatoes, spreading them evenly. Pour the rest of the sauce over the top and press the potatoes gently to ensure everything is coated.
Bake: Cover the dish with foil and bake for 40 minutes.
Add cheese: Remove the foil, sprinkle the grated reduced-fat cheddar over the top, and return to the oven for 10 minutes or until the cheese is golden and bubbling.
Serve: Allow the gratin to rest for 5 minutes before serving. Enjoy as a delicious side dish with roasted meats or a hearty vegetarian main.