Cook pasta according to package directions, omitting salt and fat.
While pasta cooks, melt butter in a medium saucepan over medium heat. Add flour; cook 1 minute, stirring constantly with a whisk. Gradually add the milk, stirring constantly with a whisk. Increase heat to medium-high; bring to a boil, stirring constantly. Reduce heat; simmer 3 minutes or until sauce thickens slightly, stirring frequently. Remove from heat, and stir in cheese, salt, and pepper. Combine the sauce, pasta, and spinach, tossing gently to coat.