In 2-quart saucepan, melt butter over medium heat. Add leeks; cook about 3 minutes, stirring frequently, until softened. Remove from heat.
Line bottom of ungreased 13x9-inch (3-quart) glass baking dish with half of the bread slices. Sprinkle with chicken and dill weed; layer with leeks. Sprinkle 2 cups of the cheese over leeks. Top with remaining bread slices; sprinkle with remaining 1 cup cheese.
In medium bowl, mix remaining ingredients; pour over bread mixture. Cover tightly; refrigerate at least 4 hours but no longer than 24 hours.
About 1 hour 15 minutes before serving, heat oven to 325°F. Uncover dish; bake 1 hour to 1 hour 5 minutes or until knife inserted in center comes out clean. Let stand 10 minutes before serving.