Sweet Potato Parmesan Gratin

Bahan-bahan (6 porsi)

• 2 organic sweet potatoes ((sliced in thin rounds))
• 1 small red or yellow potato
• ~1/4 tsp each salt and freshly ground pepper
• 1 tsp thyme ((roughly chopped // fresh or dried))
• 3/4 -1 cup Parmesan cheese ((finely grated*))
• 2 cups low-fat milk ((I used 2%)*)