Melt butter in a 3-quart saucepan over medium heat; add leeks, and sauté 5 minutes or until tender (do not brown). Add garlic; sauté 1 minute.
Whisk in flour; cook, whisking constantly, 1 minute. Gradually whisk in milk. Stir in 2 teaspoons salt and next 3 ingredients.
Cook over medium heat, whisking constantly, until mixture thickens. Remove from heat; stir in cheese until melted and smooth.
Layer half of potatoes in a lightly greased 13- x 9-inch baking dish; pour half of sauce evenly over potatoes in dish. Repeat layers, ending with sauce.
Bake, uncovered, at 375° for 50 minutes or until potatoes are golden brown and fork tender, shielding with aluminum foil to prevent excessive browning, if necessary. Remove from oven, and let stand 10 minutes.