Preheat oven to 350°F (175°C).
Cook the spaghetti according to package instructions until al dente. Drain and set aside.
Sauté aromatics: In a large skillet, melt the butter over medium heat. Add the onion and cook until soft and translucent (about 4–5 minutes). Add garlic and cook for another 30 seconds.
Mix the sauce: Stir in the cream of chicken soup, cream of mushroom soup, sour cream, and chicken broth. Stir until smooth and combined. Add 1 cup of shredded cheese, and mix until melted and creamy.
Season with paprika, salt, pepper, and red pepper flakes if using.
Combine everything: Add the cooked spaghetti and shredded chicken to the sauce. Toss until everything is evenly coated.
Transfer to a baking dish: Pour the mixture into a greased 9x13-inch baking dish. Top with the remaining 1 cup of cheddar cheese.
Bake for 20–25 minutes, or until hot and bubbly.
Garnish with chopped parsley and serve warm.