How to Make It
Heat oven to 375° F. Melt the butter in a medium saucepan over medium heat. Sprinkle with the flour and cook, whisking, for 30 seconds. Slowly whisk in the milk, 1½ teaspoons salt, and ¼ teaspoon pepper and bring to a boil. Remove from heat and stir in the Cheddar until melted.
In a small bowl, combine the bread pieces, oil, and ¼ teaspoon each salt and pepper.
Place the leeks in a single layer in the bottom of an 8-by-8-inch square or some other 2-quart baking dish. Cover with the milk mixture and sprinkle with the bread mixture. Cover the dish with foil and bake until the leeks are tender and the filling is bubbling, 25 to 35 minutes. Remove the foil and bake until the top is golden brown, 8 to 10 minutes more.