Preheat oven to 375°F (190°C). Grease a 9x13 inch baking dish.
Cook egg noodles according to package directions until al dente. Drain and set aside.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add onions and celery, cooking until softened, about 5 minutes.
Add flour to the skillet and stir for 1 minute to create a roux.
Gradually whisk in chicken broth and milk. Simmer, stirring constantly, until the sauce thickens, about 5 minutes.
Remove from heat and stir in sour cream, garlic powder, onion powder, sage, salt, and pepper.
In a large bowl, combine the cooked noodles, chicken, frozen peas and carrots, and the creamy sauce. Mix well.
Transfer the mixture to the prepared baking dish. If using cheese, sprinkle it over the top now.
In a small bowl, mix crushed Ritz crackers with 2 tablespoons of melted butter. Sprinkle this mixture evenly over the casserole.
Bake for 25-30 minutes, until the top is golden brown and the casserole is bubbling.
Let cool for 5-10 minutes before serving.