Preheat the 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, half the 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, remaining 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.