1 Preheat the oven to 200°C (180° fan) | 400F | gas 6.
2 Brush the casserole dish with butter. Place alternating layers of potato and zucchini in it. Lightly season each layer with salt and pepper.
3 Mix together the milk, cream, eggs, thyme and crushed garlic. Season with salt, pepper and nutmeg.
4 Pour enough of the milk and cream mixture over the gratin to just cover it.
5 Sprinkle with the Parmesan and bake in the preheated oven for 45-50 minutes.