Preheat oven to 350 degrees F. Coat a 9x13 baking dish with cooking spray.
Cook brown rice spaghetti according to package instructions, drain and set aside.
In a large saucepan, heat the olive oil over medium heat. Saute the onion for 2 minutes; add the mushrooms and cook until tender. Add the garlic and cook for another minute.
Stir in flour, pepper, and nutmeg until the onions and mushrooms are evenly coated.
Next, add the broth and milk and stir well, scraping the pan to loosen any bits stuck to the pan. Continue to cook over medium-low heat until the mixture is thickened and bubbly.
Add the chicken and 1 tablespoon of nutritional yeast and stir well. Finally, add the cooked pasta and stir to combine.
Pour mixture into prepared baking dish. Bake in preheated oven for 15-20 minutes.
Sprinkle with Italian seasoning if desired. Serve immediately.