1In large skillet, heat butter over medium heat. Cook mushrooms, onions and spinach in butter until vegetables are softened and spinach is wilted, about 4 to 5 minutes.
2Stir in garlic. Add the flour and stir for 1 minute until the pasty taste cooks off. Add the half-and-half and stir slowly as sauce thickens. Add the chicken to skillet; cook until hot, stirring occasionally. Stir in Parmesan cheese. Season with salt and pepper to taste.
3Add cooked spaghetti and reserved pasta water. Stir to combine.