Heat the butter in a large saucepan over medium heat. Once melted, sprinkle in the flour and whisk until combined. Continue to whisk until the roux is golden brown and smells like shortbread, about 1 minute.

Remove from heat and whisk in 1/3 of the milk slowly until a smooth paste forms. Whisk in the remaining milk, and place back over medium heat. Bring to a boil, whisking constantly, about 10 minutes. The sauce will have reduced by about 1/3 and will coat a wooden spoon. Season with salt and pepper, to taste, and pour into a large bowl and allow to cool completely.

Once the sauce has cooled, add the elbow macaroni and stir until completely coated with the sauce. Cover with plastic wrap and chill in the refrigerator overnight until the pasta has softened completely.

Heat the olive oil in a large skillet over medium heat. Saute the mushrooms until browned and softened, about 5 minutes. Add the thawed peas and saute 1 minute more. Remove from heat and store in an air tight container in the fridge until the next day.

Add the chicken, mushrooms, peas, and 1 1/2 cups of the cheese to the pasta mixture and stir to combine. Pour into the prepared casserole dish and spread evenly. Sprinkle the remaining 1/2 cup of cheese on top.

In a small bowl, combine the panko breadcrumbs with the melted butter until coated, and sprinkle over the top of the casserole. Bake for 30-40 minutes or until bubbly and golden brown. Let stand for about 10 minutes, serve, and enjoy!