Preheat the oven to 180ºC (350°F).
Cook the pasta according to package instructions minus 3 minutes (it should be a bit raw still). Rinse in cold water and reserve.
12 ounces egg noodles
Melt the butter at medium heat in a pot and add the onion and celery, and cook until softened, about 5 minutes.
¼ cup unsalted butter, 2 cups onions, 1 cup celery
Stir in the flour and cook over low heat for about 3 minutes, stirring constantly.
¼ cup all-purpose flour
Gradually add the milk, cream and stock to the pot. Stir in the oregano and mustard.
1 ½ cups milk, ¼ cup heavy cream, 2 cups chicken stock, 2 teaspoons dry oregano, 2 tablespoons whole-grain mustard
Bring to a simmer and cook over medium-low heat, stirring frequently, for about 5-8 minutes.
Add the grated cheese, stirring until the cheese melts. Mix in the cooked chicken and season with salt and pepper to taste.
6 ounces Gruyere cheese, 3 cups cooked chicken, Salt and pepper
Mix the pasta with the chicken sauce. Adjust seasoning as needed.
Mix the panko with melted butter.
½ cup panko breadcrumbs, 1 tablespoon butter
Transfer the mixture to a 9x13 casserole dish. Top with the panko mixture and bake for 10 minutes, or until golden brown.
Garnish with parsley and serve.
Freshly chopped parsley