Heat oven to 350°. Grease an 8" square baking dish with butter; set aside. Melt butter in a 6-qt. saucepan over medium-high heat. Add garlic and onion; cook, stirring occasionally, until golden, 8–10 minutes. Add flour; cook, stirring, until smooth, 2 minutes. Add cream, milk, thyme, salt, and pepper; bring to a boil. Add potatoes; cook, stirring occasionally, until potatoes are just tender, 8–10 minutes. Transfer mixture to prepared baking dish. Using a spoon, press mixture down; sprinkle with paprika. Bake until golden brown and a knife inserted into middle of potatoes slides in easily, about 40 minutes.