Cook spaghetti according to package directions, adding oil and salt. Drain.
In large Dutch oven, melt butter over medium heat. Add onion and garlic. Cook, stirring for 2 minutes. Add soup, broth, salt and 1 1/2 cups of the cheese. Cook and stir until smooth and melted. Add spaghetti and chicken, mixing well. Transfer to a 2-quart baking dish. Cover with foil and bake for 30 minutes.
Remove foil and sprinkle with rest of cheese. Bake until cheese melts, then sprinkle with parsley and pimento.