Preheat and Prep: Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with butter or cooking spray and set aside.

Cook the Pasta: Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package directions until al dente, usually 8-10 minutes. Drain well, reserving 1 cup of the pasta water, and set aside.

Sauté the Vegetables: In a large Dutch oven or deep skillet, melt the butter over medium heat. Add the diced onion and bell pepper, cooking for 4-5 minutes until softened and fragrant. Add the minced garlic and cook for another minute, stirring constantly to prevent burning.

Build the Sauce Base: Pour in the chicken broth and bring to a gentle simmer. Add both cans of Rotel tomatoes (with their juice) and the cream of chicken soup. Stir well to combine all ingredients.

Season the Sauce: Sprinkle in the cumin, chili powder, and paprika. Season with salt and pepper to taste. Stir and let the mixture simmer for 3-4 minutes to allow the flavors to meld together.

Melt the Velveeta: Reduce heat to low. Add the cubed Velveeta cheese to the sauce, stirring constantly until it's completely melted and the sauce is smooth and creamy. This should take about 3-5 minutes. The sauce should be glossy and silky.

Add the Cheddar: Stir in 1 cup of the shredded cheddar cheese (reserving the remaining ½ cup for topping), stirring until melted and fully incorporated into the sauce.

Combine Everything: Add the cooked, drained spaghetti and shredded chicken to the cheese sauce. Toss everything together gently but thoroughly until the pasta and chicken are well coated with the creamy sauce. If the mixture seems too thick, add some of the reserved pasta water, a few tablespoons at a time, until you reach your desired consistency.

Transfer and Top: Pour the chicken spaghetti mixture into the prepared 9x13-inch baking dish, spreading it out evenly. Sprinkle the remaining ½ cup of shredded cheddar cheese evenly over the top.

Bake: Place the dish in the preheated oven and bake for 20-25 minutes, until the cheese on top is melted and the edges are bubbling. For a golden, crispy top, turn on the broiler for the last 2-3 minutes—watch it closely to prevent burning!

Rest and Serve: Remove from the oven and let the casserole rest for 5 minutes before serving. This helps it set and makes it easier to serve. Garnish with fresh cilantro, sliced jalapeños, or a dollop of sour cream if desired. Enjoy!