Preheat and Prep the Pasta: Start by preheating your oven to 350°F (175°C). While the oven is heating, bring a large pot of water to a boil. Add your spaghetti and cook it just until al dente—this usually takes about 8–9 minutes, depending on the brand. You don’t want it too soft because it will continue cooking in the oven. Drain the pasta and set it aside while you prepare the sauce.
Melt the Cheese and Create the Sauce: In a medium saucepan over medium heat, combine the cream of chicken soup, the full can of Rotel (undrained), and the cubed processed cheese. Stir continuously as the cheese melts and blends into the soup and tomatoes. This will take about 5–7 minutes. You’ll end up with a smooth, creamy mixture. Be careful not to let it boil—low and slow helps prevent curdling or clumps.
Combine Pasta, Chicken, and Extras: In a large mixing bowl or directly in the baking dish (if it's deep enough), add your drained pasta, cooked chicken, olives, and mushrooms. Pour the cheese and soup mixture over everything. Use a large spoon or spatula to gently mix everything together until evenly coated. Now’s a great time to season it—add salt and pepper to taste, but remember that the soup and cheese already contain sodium.
Transfer and Bake: Pour the fully mixed casserole into your greased 9x13-inch baking dish. Spread it evenly across the dish so it bakes uniformly. If you want, sprinkle a little extra cheese on top or even some crushed crackers for a crispy topping. Cover the dish loosely with foil and bake for 20 minutes, then remove the foil and bake uncovered for an additional 10 minutes. You’re looking for a bubbly surface with golden edges.
Rest and Serve: Once it’s baked, let the casserole sit for 5 to 10 minutes before serving. This rest time allows the sauce to thicken slightly and makes it easier to cut or spoon out clean servings. I usually garnish it with a sprinkle of fresh parsley or chopped green onions to add color and freshness.