Preheat and Prep: Heat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Cook the Pasta: Bring a large pot of salted water to a boil. Cook the spaghetti 1–2 minutes shy of al dente.
Drain and set aside. Do not overcook—the pasta will finish in the oven.
Sauté Aromatics: In a large skillet, warm butter or oil over medium heat. Add onion (and bell pepper if using) and cook until softened, about 4–5 minutes. Stir in garlic for 30 seconds until fragrant.
Build the Sauce Base: Add the Rotel (with juices), chicken broth, and cream of chicken soup to the skillet.
Stir until smooth and bubbling gently.
Melt in the Creaminess: Reduce heat to low. Add cream cheese in small pieces, stirring until fully melted and the sauce is smooth. Season with 1 teaspoon salt, 1/2 teaspoon black pepper, 1/2 teaspoon paprika, and 1/2 teaspoon garlic powder.
Adjust to taste.
Add the Chicken and Cheese: Stir in the cooked chicken and 2 cups shredded cheddar until just melted.
Combine With Pasta: Toss the spaghetti into the sauce, coating every strand. If it looks too thick, splash in a bit more broth or a touch of milk to loosen.
Transfer and Top: Pour the mixture into the prepared baking dish. Sprinkle a generous handful of cheddar on top.
For crunch, add a few crushed tortilla chips.
Bake: Bake uncovered for 20–25 minutes, until bubbling around the edges and the top is lightly golden.
Rest and Serve: Let it rest 5 minutes so the sauce sets slightly. Garnish with chopped green onions or cilantro if you like. Serve warm.