- Preheat oven to 375°F (190°C).
- Peel potatoes; slice into rounds no thicker than 1/8 inch (0.3 cm). In a medium saucepan, melt butter over medium heat; add garlic and salt. Stir 1 min. Whisk in flour until combined. Gradually add cream, whisking until combined. Bring to a boil, whisking occasionally until thickened, for 2 to 3 min.
- Layer half the potatoes in an 8 x 8-inch (20 x 20 cm) baking dish. Scatter with half the thyme. Top with half the cream sauce. Repeat with remaining potatoes; drizzle remaining cream mixture over top. Press potatoes to even out layers and coat in cream. Cover with foil. Bake for 30 min.
- Sprinkle with cheddar cheese and remaining thyme. Bake uncovered for 15 min until bubbly and top is a deep golden brown. Let stand for 10 min.