Preheat your oven to 375°F (190°C). Lightly grease a baking dish with cooking spray or olive oil.
Thinly slice the root vegetables using a mandoline or sharp knife for even cooking.
Arrange a layer of vegetable slices in the dish, then top with some minced garlic, onion, salt, pepper, and herbs. Repeat the layers until all vegetables are used.
In a bowl, mix the milk with half of the Parmesan and Gouda cheeses. Pour this mixture over the layered vegetables, ensuring it seeps in between the layers.
Sprinkle the remaining cheese on top for a flavorful crust.
Cover with foil and bake for about 40 minutes. Remove the foil and continue to bake for another 20-30 minutes, or until the top is golden and the vegetables are tender.
Allow it to cool slightly before serving to help the gratin set and make it easier to cut. Enjoy!