Preheat oven to 180 °C or 160 °C fan. Lightly grease a 7-cup ovenproof dish. Layer 1/3 of potatoes over base. Top with half the onion, garlic and thyme. Season with salt and pepper. Pour over 1/3 of combined milk and cream.

Repeat layers. Finish with remaining potato and pour remaining milk and cream over. Brush a sheet of foil with oil and place oil-side down over the potato. Seal tightly. Bake for 45 mins. Uncover and brush top with melted butter. Cook a further 45 mins until tender and golden brown. Sprinkle with parmesan for last 15 mins of cooking