Add the thinly sliced potato slices in 3 rows, into a deep sided rectangle baking dish. Sprinkle over a little of the tasty cheese and a couple of spoonfuls of the onion sour cream mixture. Repeat the process to form 3-4 layers in total. Pour the remaining onion cream mixture followed over the top by the parmesan cheese and any left over tasty cheese. Sprinkle over the top the rest of the thyme.
Bake in the oven for 50-55 minutes or until the top is golden and crispy and the potato are soft when pricked with a fork.