Melt butter in a large skillet over medium heat. Add flour and stir to combine. Cook the flour for 1 minute then add the milk whisking as you pour. Continue to whisk as the mixture comes to a boil. Turn the heat to medium low and continue to cook for 3 minutes. Transfer to a bowl and set aside. Wipe out the skillet.
Heat oil in the skillet over medium heat then sauté onions and celery. Cook until onion is translucent and celery is soft. Add garlic and green beans and cook an additional 2 minutes.
Transfer vegetable mixture to large mixing bowl then add chicken, pasta, white sauce, and cheese. Combine thoroughly.
Pour mixture into a 9" square casserole dish and sprinkle with einkorn breadcrumbs.
Bake uncovered, 30-35 minutes until the center is hot and sides are bubbly.
Turkey is also a great alternative to chicken for this hearty meal!