Cook the chicken in lightly salted water or chicken broth until the thickest part reaches 165°F, about 12–18 minutes depending on thickness. Shred the chicken while warm and reserve 1 cup of the cooking liquid; rotisserie chicken may be used instead.

Heat the oven to 350°F and lightly grease a 9-by-13-inch baking dish. Cook the spaghetti in salted water until just shy of al dente, usually 1–2 minutes less than the package directions, then drain.

Melt the butter in a large skillet over medium heat. Add the onion and green bell pepper and sauté for 5–7 minutes, until softened, then stir in the garlic and cook for 30 seconds.

Stir in both condensed soups, the undrained Rotel tomatoes, chicken broth, salt, pepper, and optional paprika. Heat gently until the sauce is smooth and hot, taking care not to let it boil vigorously.

Add 1 cup of the shredded cheddar cheese and stir until melted into the sauce. The cheese should make the mixture creamy while the broth keeps it loose enough for baking.

Add the drained spaghetti and shredded chicken to the skillet. Toss carefully until the pasta and chicken are evenly coated with the creamy sauce.

Transfer the mixture to the prepared 9-by-13-inch baking dish and spread it into an even layer. Sprinkle the remaining 1 cup of cheddar over the top so every portion gets a golden, cheesy finish.

Bake the casserole uncovered at 350°F for 20–25 minutes, until it is bubbling around the edges and the cheese has melted. Let it rest for 5–10 minutes before serving so the sauce can settle.