Wash and dry potatoes. Prick potatoes several times with the tines of a fork.
Bake potatoes in a 425° for 60 minutes until soft and skin is crisp.
Cool potatoes until they are cool enough to handle.
Cut the top ⅓ off each potato. Using a spoon scoop out the flesh inside leaving about a ¼ inch of flesh attached to the potato skins, forming "bowls".
In a mixing bowl, combine the scooped potato flesh with ¼ cup milk, 2 tbsp butter and 1 tsp salt into mashed potatoes. Set aside.
Preheat oven to 350°F.
In a skillet, heat oil over medium heat. Sauté onion until soft and translucent.
Add the ground beef, onion and celery to the skillet and cook until beef is browned; breaking meat apart into crumbles as it cooks.
Once beef is cooked through and onion and celery are softened, sprinkle flour evenly over the meat mixture and stir to combine.
Add the beef broth, Worcestershire sauce, sliced mushrooms, garlic powder, salt and pepper to the skillet.
Stir to combine.
Increase heat to medium high and bring the mixture to a simmer.
Simmer until the meat mixture thickens. Remove from heat.
Divide the meat mixture evenly among the hollowed out potato skins.
Top each filled potato with a sprinkle of grated Parmesan cheese.
Use a spoon and top each potato with a layer of mashed potatoes to cover the meat filling completely.
Top each potato with a little more grated Parmesan cheese.
Bake for 30-45 minutes, or until the filling is piping hot and the tops of the potatoes turn golden brown.