Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1/4 cup. Set aside.
Melt butter in a large nonstick skillet over medium heat. Add leek; cook, stirring frequently, 6 minutes or until lightly browned. Sprinkle with flour; stir well. Add milk; increase heat to medium-high, and cook, stirring constantly, until mixture comes to a simmer. Reduce heat, and simmer 1 minute, stirring constantly. Remove from heat; add 1/2 cup cheese and next 6 ingredients. Stir well. Spoon mixture into a 10-inch quiche dish or 9- inch pie plate coated with cooking spray. Combine remaining cheese and breadcrumbs; sprinkle over gratin. Bake at 375° for 30 minutes or until set. Cool in dish 5 minutes on a wire rack.