Peel potatoes and soak in cold water. Rinse thoroughly and cut thick wide slices so that you can then cut them down into thin french fry sticks.
Rinse the sticks again and dry carefully with a clean cloth. Place in a large bowl and toss with olive oil until coated.
Sprinkle with the Parmesan and the thyme and place the oiled frites into a freezer bag. Close tightly and shake thoroughly to disperse the seasonings evenly. Open the bag and add salt and pepper and then close and shake thoroughly again.
Spread out onto a baking sheet covered with parchment paper or a silicone liner.
Cook in preheated oven 20-30 minutes. Check the seasonings at the end of the cooking time and enjoy immediately.