Place bread in a food processor; pulse 10 times or until coarse crumbs measure 4 cups; set aside.
Heat a large nonstick saucepan over medium-high heat. Coat with cooking spray. Add pancetta, onion, and garlic; sauté 5 minutes or until onion is tender. Lightly spoon flour into a dry measuring cup; level with a knife. Place flour in a small bowl; gradually add 3/4 cup milk, stirring until smooth. Add flour mixture, remaining 3 cups milk, and half-and-half to pan; bring to a boil, stirring constantly. Reduce heat, and simmer 2 minutes or until thick, stirring constantly. Add spinach; cook 1 minute. Remove from heat. Stir in cheese, salt, and pepper, stirring until cheese melts.
Place pasta in a large bowl. Add sauce mixture to pasta; toss well. Spoon mixture into a 13 x 9-inch baking dish coated with cooking spray. Combine breadcrumbs and butter; toss well. Sprinkle over pasta mixture.
Bake at 425° for 6 minutes or until crumbs are lightly browned.