1. Preheat oven to 180c or 160c fan.
2. Lightly grease a 7 cup ovenproof dish.
3. Layer 1/3 of potatoes over base.
4. Top with half the onion, garlic and thyme. Season with salt and pepper.
5. Pour over 1/3 of combined milk and cream.Repeat layers.
6. Finish with remaining potato, and pour remaining milk and cream over.
7. Brush a sheet of foil with oil and place oil-side down over the potato. Seal tightly.
8. Bake for 45 mins. Uncover and brush top with melted butter.
9. Cook a further 45 mins until tender and golden brown.
10. Sprinkle with parmesan for last 15 mins of cooking.