1. Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add onion and bell pepper; saute 5 minutes or until tender.
2. Place onion mixture and macaroni in a 3-quart electric slow cooker coated with cooking spray.
3. Place flour and next 4 ingredients in a small bowl; gradually add milk, stirring with a whisk until well blended.
4. Add milk mixture, cheeses, and pimiento to slow cooker; stir well.
5. Cover with lid; cook on high-heat setting 1 1/2 hours or until thick and creamy, stirring after 1 hour.