Heat oven to 400°F.
Grease 13×9-inch (3-quart) glass baking dish or 3-quart casserole with the butter.
In large bowl, mix flour, thyme, salt and pepper.
Gradually add whipping cream, stirring with whisk until smooth.
Peel potatoes; cut into ¼-inch slices, then cut slices into quarters.
Add potatoes, garlic and leek to cream mixture, stirring to coat.
Spoon potato mixture into baking dish.
Cover with foil; bake 30 minutes.
Uncover; bake 25 minutes longer or until top is lightly browned and potatoes are tender.
Sprinkle with cheese; bake 5 minutes longer or until cheese is melted.
Serve immediately.