Prepare the Pasta: Bring a large pot of salted water to a rolling boil. Add the elbow macaroni and cook for 1-2 minutes less than the package directions for al dente. The goal is for the pasta to be very firm, as it will continue to cook in the slow cooker.
Stop the Cooking: Immediately drain the pasta and rinse it thoroughly under cold water. This step is crucial as it stops the cooking process and removes excess starch, preventing the final dish from becoming mushy.
Combine the Ingredients: Spray the insert of a 4-quart slow cooker with non-stick cooking spray for easy cleanup. Add the rinsed pasta, cream of chicken soup, shredded cheddar and Gruyère cheeses, mayonnaise, sour cream, onion powder, mustard powder, and black pepper.
Mix Thoroughly: Stir everything together until the pasta is evenly coated with the sauce ingredients. This will ensure every bite is cheesy and flavorful.
Slow Cook to Perfection: Secure the lid and cook on the HIGH setting for 2 hours or on the LOW setting for 3 hours. To prevent sticking and ensure even heating, stir the mixture once or twice during the cooking process. The mac and cheese is ready when the sauce is hot, bubbling, and has reached a luscious, creamy consistency.