Ingredients:

Instructions:

  1. Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.
  2. Cook the spaghetti according to package directions, subtracting 2 minutes from the suggested time to ensure it is very al dente. Drain well and set aside.
  3. In a large skillet over medium heat, melt 2 Tbsp of butter. Add mushrooms and sauté until moisture evaporates and they begin to brown. Remove and set aside.
  4. In the same skillet, melt the remaining 2 Tbsp of butter over medium heat. Whisk in the flour and cook for 1–2 minutes, stirring constantly, to form a smooth roux.
  5. Slowly whisk in the warm milk until fully incorporated, then gradually whisk in the chicken broth. Bring mixture to a simmer, whisking constantly, until the sauce thickens enough to coat the back of a spoon.
  6. Remove the sauce from the heat. Stir in salt, pepper, nutmeg, 1/2 cup Parmesan, and the Gruyère/Cheddar until completely smooth.
  7. Gently fold the cooked spaghetti, shredded chicken, and sautéed mushrooms into the cheese sauce until everything is evenly coated.
  8. Pour the entire mixture into the prepared casserole dish.
  9. In a small bowl, combine the Panko breadcrumbs, 2 Tbsp melted butter, and the remaining 1/4 cup Parmesan. Sprinkle this topping evenly over the casserole.
  10. Bake for 30–35 minutes, or until the filling is bubbling and the topping is golden brown and crisp. Let rest for 5 minutes before serving.