Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook the spaghetti according to package directions until al dente. Drain well and set aside.
Step 3: While the pasta cooks, heat olive oil in a large skillet over medium heat. Add the chopped onion and sliced mushrooms; sauté for 5-7 minutes until softened and lightly browned. Add minced garlic and cook for another minute until fragrant. If using, stir in the dry sherry and let it simmer for 2-3 minutes, allowing the alcohol to cook off. Remove vegetables from the skillet and set aside.
Step 4: In the same skillet (or a large pot), melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 5: Gradually whisk in the chicken broth and milk until the sauce is smooth and thickened. Stir in the cream of chicken soup until well combined. Bring to a gentle simmer, then reduce heat to low.
Step 6: Stir in the cooked chicken, sautéed vegetables, frozen peas (if using), 1/2 cup Parmesan cheese, and 1/4 cup fresh parsley into the sauce. Season with salt and pepper to taste.
Step 7: Add the cooked spaghetti to the sauce mixture and toss gently until all the pasta is evenly coated.
Step 8: Pour the mixture into the prepared baking dish. Sprinkle with additional Parmesan cheese and, if desired, Panko breadcrumbs for a crispy topping.
Step 9: Bake for 25-30 minutes, or until bubbly and golden brown on top. Let stand for 5 minutes before serving. Garnish with fresh parsley.