Preheat the oven to 350°F. Coat a 9×13 inch baking dish with nonstick spray and set aside. Measure out the Greek yogurt and let it sit at room temperature while you proceed with the recipe.

Cook the noodles 1 minute short of al dente according to package instructions (for my package this was 4 minutes). They should still be slightly too chewy to eat, as they will continue to cook in the oven. Drain, toss with a bit of oil to prevent sticking, and set aside.

Meanwhile, melt the butter in a Dutch oven or similar large, deep pot over medium heat. Add the onion and celery. Stir and cook until softened and the onion is turning translucent, about 6 minutes.

Sprinkle the flour, garlic powder, salt, thyme, sage, and pepper over the top. Cook, stirring so that no dry bits of flour remain, for 1 full minute.

Working a flew splashes at a time, whisk in the broth and milk, stirring until smooth. Bring the liquid to a simmer over medium high then let cook, stirring often, until the mixture thickens enough to coat the back of a spoon, about 10 to 15 minutes once simmering has begun. Remove from the heat.

Off the heat, stir in the peas and carrots (this will help cool the sauce so the yogurt doesn't curdle), then the Greek yogurt and cheese until smoothly combined.

Fold in the chicken and noodles, then transfer to the prepared baking dish. Smooth the top.

With your fingers, crush the crackers over the casserole to create a crunchy topping.

Bake until the casserole is hot and bubbly and the crackers are golden, about 25 to 30 minutes. If desired, sprinkle with chopped fresh parsley. Let cool a few minutes, then serve.