Sauté onion and garlic in butter. Add chili powder, chicken broth, rotel and cheese. Cook over low heat, stirring frequently, until cheese is melted. Add chicken and spaghetti. Mix together and place in large casserole dish. Cover and freeze. To prepare, thaw to room temperature. Heat in covered casserole dish at 350 for 30 – 40 minutes or until hot and bubbly.