Preheat oven to 350 degrees F. Spray a 3-quart casserole dish with the cooking spray.
Fill a large saucepan with water and boil the potatoes until fork tender, about 10 minutes. Drain and mash the potatoes with the milk, sour cream, pepper, and salt. Set aside.
In another pan, saute the onion, beef, celery, and garlic until the beef is cooked through and the onions are translucent. Remove from the skillet; set aside.
Combine the beef stock, flour, oats, and Worcestershire sauce in skillet to make the gravy. Heat to boiling, stirring until thickened.
Place the beef mixture in the bottom of the prepared casserole dish. Top with the green beans and the corn. Pour the gravy over the top. Spoon the mashed potatoes on top of the gravy layer. Bake, uncovered, 30 minutes. Top with cheese; return to the oven and bake 10 to 15 minutes, or until the cheese bubbles.