Preheat oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
In a cast-iron skillet over medium-high heat, cook the beef until no longer pink, breaking up large crumbles with the back of a wooden spoon. Use a slotted spoon to transfer the beef to a paper towel-lined plate.
Reduce the heat to medium.
To the same skillet, add the onion, bell pepper, and mushroom. Cook until softened. Add the beef back into the skillet and stir to combine.
Stir in the ketchup, Worcestershire sauce, salt, and pepper. Let the mixture continue cooking while you prepare the corn starch slurry.
In a small bowl, whisk together the beef broth and corn starch. Pour the slurry into the beef mixture and stir to combine. Allow the mixture to simmer until most of the liquid has evaporated.
Stuff the mixture into the cooked shells and place into the prepared baking dish.
Top with shredded provolone cheese.
Bake 15-20 minutes, or until bubbly.
Garnish with scallions and serve.