Melt butter in a 6-quart Dutch oven or soup pot over medium high heat. Add onions and mushrooms and cook for 3 ½ to 4 minutes (or until softened), stirring frequently. Add garlic and stir an additional 30 seconds, stirring constantly.
Add chicken broth, uncooked spaghetti noodles, cooked chicken, peas, salt and pepper. Gently stir to combine. Ensure that noodles are submerged in liquid.
Bring to a moderate boil, then reduce heat as necessary to maintain a simmer (medium low to low). Cover and simmer for 20 minutes, or until most of the liquid is absorbed and noodles are tender. *
Add cream cheese and sour cream. Stir until smooth and combined.
Top with shredded Parmesan, mozzarella or cheddar cheese when serving, if desired.