Preheat the oven to 375°F.
Grease a baking dish with butter.
Mix the potatoes, milk, broth, onion, salt, pepper, and garlic in a bowl.
Pour the mixture into the baking dish and spread evenly.
In a small saucepan over medium heat, melt butter and then stir in the flour until you get a paste.
Pour the butter-flour mixture over the potatoes in the baking dish.
Bake for 45-50 minutes, or until golden brown on top.
Cover the baking dish with foil and bake for an additional 15 minutes or until potatoes are tender. Serve warm.