Preheat oven to 350 degrees F. Lightly spray medium casserole with cooking spray; set aside.
Microwave potatoes according to package directions for 5 minutes. In a medium bowl, stir together cheddar cheese soup with evaporated milk until smooth. Place half of the potatoes in the bottom of prepared casserole. Layer with half each of pimientos, onion, cheese soup mixture, and shredded cheese. Repeat layers. Bake in preheated oven for 45 to 55 minutes or until bubbling.