Peel the potatoes completely clean.
Slice them into thin, even rounds.
Aim for about ⅛ inch thickness.
Thinner slices cook more evenly.
Place slices in cold water.
This prevents them from turning brown.
Melt butter in your saucepan over medium heat.
Add minced garlic and cook 30 seconds.
Whisk in the flour completely.
Cook for one minute, stirring constantly.
Slowly pour in milk and cream.
Whisk until the sauce thickens nicely.
Add salt, pepper, and thyme now.
Preheat your oven to 375°F first.
Grease your baking dish with butter.
Drain the potato slices thoroughly.
Layer half the potatoes in the dish.
Pour half the sauce over them.
Sprinkle one cup of cheese on top.
Repeat with remaining potatoes and sauce.
Save the last cup of cheese.
Cover the dish tightly with foil.
Bake for 45 minutes covered.
Remove foil and add remaining cheese.
Bake uncovered for 30 more minutes.
The top should turn golden brown.
Potatoes should be fork-tender throughout.