Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish.
Cook the spaghetti according to package directions until al dente. Drain and set aside.
In a large skillet, melt butter over medium heat. Add onions and green peppers; sauté until softened.
Add the cooked chicken, cream of chicken soup, Rotel, sour cream, garlic powder, salt, and pepper. Stir well to combine.
Add in the cooked spaghetti and 1 1/2 cups of shredded cheddar cheese. Mix until everything is evenly coated.
Transfer the mixture into the prepared baking dish. Top with the remaining 1/2 cup of cheese.
Bake for 25-30 minutes or until hot and bubbly.
Garnish with chopped parsley if desired and serve warm.