Preheat oven to 425°.
Arrange half of the potato slices in an 11 x 7-inch baking dish coated with cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add leek; sauté 10 minutes or until lightly browned. Stir in garlic, salt, and pepper. Spoon half of leek mixture over potato slices; top with half of tomatoes and half of cheese. Repeat layers with remaining potatoes, leek mixture, tomatoes, and cheese.
Bring milk to a boil over low heat in a skillet; pour over potato mixture. Cover and bake at 425° for 30 minutes or until potatoes are tender. Uncover; bake an additional 15 minutes or until golden brown. Let stand 5 minutes before serving.