Place bread in a food processor; pulse 10 times or until coarse crumbs measure 1/4 cup. Combine breadcrumbs and butter, stirring to coat.
Heat a large nonstick skillet over medium-high heat. Coat pan with cooking spray. Add onion; sauté 3 minutes or until tender. Add spinach, juice, nutmeg, and garlic; cook 2 minutes, stirring occasionally. Combine milk and flour, stirring well with a whisk. Gradually add milk mixture to spinach mixture, stirring constantly; bring to a boil. Add cheese, salt, and pepper; cook 30 seconds or until cheese melts, stirring constantly. Remove from heat, and stir in pasta. Spoon the mixture into a 1-quart soufflé dish coated with cooking spray. Top evenly with breadcrumb mixture.
Broil for 3 minutes or until lightly browned.