• Preheat oven to 400°F.  Grease a 9X13-inch baking dish with cooking spray, and set it aside.
• Peel the potatoes and keep them covered in water (to prevent them from browning).

Make the cream sauce

• In a saucepan melt butter. Add minced garlic and saute for 30 seconds. The garlic should be aromatic.
• Reduce heat to low. Add flour to the saucepan and saute flour for about a minute on very low heat. Make sure the flour doesn't change the colour to brown.
• Slowly add milk to the saucepan, while stirring the mix with a wooden spatula using another hand. Keep stirring until all the milk has been added to the pan and the mixture is smooth (lump free). Then stir in the chicken stock followed by onion powder, salt, pepper and thyme. Let the sauce simmer for 2-3 minutes. It will thicken further.
• Take off the saucepan from the stovetop. Add cheese and stir until the sauce is completely smooth.

Bake the potatoes

• Take out the peeled potatoes from water, using a mandoline slicer cut into 1/8th inch thickness.
• Arrange half of the potatoes in the prepared casserole dish to layers. pour 1/3rd of the sauce on top. Layer rest of the potatoes. Then pour remaining cream sauce on top. Spread the sauce evenly.
• Loosely cover the casserole dish with aluminum foil and bake in the oven for about 45 minutes.
• Remove the foil and bake uncovered for another 20-25 minutes until the potatoes are cooked through and the top looks set with a few golden sports.
• Let the casserole rest for 10 minutes, then serve.