Preheat oven to 375°.
In a 2-quart saucepan, melt butter over low heat and stir in flour to create a roux. Stir mixture for 1 minute, taking care not to brown. Whisk in milk to combine well with roux; bring to a simmer. Add cheese and garlic; continue whisking until cheese is melted completely. Add salt and pepper to suit your taste.
In a lightly oiled casserole dish (such as a 9-by 13-inch), coat the bottom of the pan with 1/3 cup of the cheese sauce. Begin layering potato slices in a circular pattern, from the edge of the pan towards the center. Sprinkle
1/3 of the chopped leeks evenly over the slices and top with ½ cup of the sauce, making sure each potato slice is covered. Repeat this 2 more times, finishing with a sprinkle of the remaining leeks and shredded cheese.
Cover the pan with foil, taking care to not have the foil touch the potato mixture. Bake 45 minutes, remove foil and bake an additional 15 minutes, or until potatoes are tender and the top is golden.