1. Preheat oven to 200C. Melt the butter in a 24cm frying pan with an ovenproof handle over a medium heat. Add the thyme and onion and cook for 5 minutes.
2. Place the potato, oil, Gruyene, mozzarella, salt and pepper in a bowl and toss to combine. Top onion with the potato mixture and bake for 45 minutes or until golden and cooked through. Turn out to serve.

Tip: This makes a great side dish for roasted meats or as a light lunch with a green salad.