Preheat oven to 350°.
Place bread in a food processor, and pulse 10 times or until coarse crumbs measure 1/2 cup.
Melt butter in a large saucepan over medium heat. Add onions and garlic; cook 1 minute. Add flour; cook 1 minute, stirring constantly. Gradually add milk, salt, and pepper, stirring constantly with a whisk until blended. Bring to a boil; cook until thick (about 2 minutes). Add cheeses; stir until melted.
Add spinach and macaroni to cheese sauce, stirring until well blended. Spoon mixture into a 2-quart baking dish coated with cooking spray. Sprinkle with breadcrumbs. Bake at 350° for 15 minutes or until bubbly.