Preheat your oven to 350°F (175°C).
Grease a baking dish with butter or non-stick spray.
In a saucepan, melt butter over medium heat and whisk in the flour. Cook for 1-2 minutes until bubbly.
Gradually whisk in milk and cream. Cook, stirring constantly, until the mixture thickens.
Stir in shredded cheese, garlic powder, salt, pepper, and thyme. Mix until the cheese is fully melted.
Layer half of the sliced potatoes in the prepared baking dish. Pour half of the cheese sauce over the potatoes.
Repeat with the remaining potatoes and cheese sauce.
Cover with foil and bake for 40 minutes. Remove the foil and bake for an additional 15-20 minutes or until the top is golden and bubbly.
Allow to cool slightly before serving.