Preheat your oven to 350°F. Butter a 9x13-inch baking dish and set it aside.
Boil the spaghetti in salted water until al dente according to package instructions. Be sure the pasta still has some bite to it so that it does not get mushy in the oven. Drain the pasta, rinse it in cold water to stop the cooking process, and set it aside.
Meanwhile, melt 2 tablespoons of butter in a Dutch oven or stockpot over medium-high heat. Add the onion and cook for 2-3 minutes until translucent. Add the garlic and cook for another minute until fragrant.
Stir in the cream of mushroom soup, cream of chicken soup, cream cheese, 1 cup of cheddar cheese, Rotel tomatoes, shredded chicken, and seasonings. Add salt and cracked black pepper to taste. Cook, while stirring, until the cheeses are melted. Stir in the chicken broth until the sauce is smooth.
Add the spaghetti to the pot and give everything a good stir to combine. Transfer the chicken spaghetti to the buttered baking dish, top with the remaining cheese, and bake it for 20 to 25 minutes until bubbly and golden brown. Garnish with fresh chopped parsley if desired. Allow the chicken spaghetti to cool for about 10 minutes and enjoy!