Method:
Preheat oven to 375°F. Spray a 2-quart casserole dish with cooking spray; set aside.
Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 5 minutes; drain well and transfer to a large bowl.
Meanwhile, heat butter in a large skillet over medium high heat. Add onions, mushrooms, salt and pepper and cook, stirring occasionally, until softened, about 5 minutes. Stir in peas and cook 2 minutes more. Transfer to bowl with pasta and set aside.
Heat oil in a small saucepan over medium heat. Whisk in flour until smooth then slowly whisk in broth and cook, stirring constantly, until thickened. Stir in milk, nutmeg, salt and pepper and cook 1 minute more. Add milk mixture to bowl with pasta and mushroom mixture along with chicken, 4 tablespoons of the cheese and 2 tablespoons of the chives and toss gently to combine. Transfer to prepared dish, sprinkle with remaining 2 tablespoons cheese and 2 tablespoons chives and bake until hot throughout and golden brown on top, about 30 minutes.