Place bread in a food processor; pulse 10 times or until coarse crumbs form to measure 2/3 cup. Set aside.
Combine 1 1/2 cups milk and next 6 ingredients (1 1/2 cups milk through garlic) in a saucepan, and bring to a boil over medium heat. Remove from heat; cover and let stand 30 minutes. Strain milk mixture through a sieve into a large bowl; discard solids.
Preheat oven to 350°.
Place flour in a small bowl; gradually add 1/2 cup milk, stirring with a whisk until blended. Add flour mixture, mustard, salt, and red pepper to milk mixture, stirring well with a whisk. Stir in macaroni and cheeses; spoon into an 8-inch square baking dish coated with cooking spray. Top with breadcrumbs. Bake at 350° for 25 minutes or until lightly browned.