Prep the Pan and Oven: Grease a 2-quart baking dish and preheat the oven to 350 F.
Cook the Bacon: Arrange the bacon in a large skillet and cook over medium heat until crisp; drain on paper towels and set aside.
Cook the Beef: Cook the beef in the bacon drippings until browned, stirring to break it up. Stir in the flour, blending thoroughly, and cook for 1 minute.
Add the Vegetables: Stir in the onion, mushrooms, carrot, and celery, along with the beef broth and Worcestershire sauce. Add the thyme and pepper. Bring to a boil over high heat, then reduce heat to low and simmer, uncovered, for about 15 minutes or until vegetables are tender. Stir in the reserved bacon and transfer to the prepared baking dish.
Assemble and Bake: Combine the hot mashed potatoes with 1/2 cup of cheese and spread evenly over the beef mixture. Top with the remaining cheese and sprinkle with a bit of paprika. Bake, uncovered, for 20 to 25 minutes or until hot. If desired, broil for a few minutes to brown the potatoes.
Serve: Spoon onto a plate and serve with a salad or fresh vegetables on the side.