Preheat the oven to 350 degrees F (175 degrees C). Lightly grease a 2-quart baking dish with cooking spray.

Heat a large skillet over medium-high heat. Cook and stir ground beef, onion, and celery in the hot skillet until beef is browned and crumbly. Drain and discard grease.

Pour beef mixture into the prepared baking dish. Add condensed soup and thyme; stir well. Spread mashed potatoes evenly on top.

Bake in the preheated oven for 20 minutes. Sprinkle with cheese and continue baking until cheese has melted, about 5 minutes more.