Preheat oven to 350°F and set aside a 9x13 baking dish.
In a large pan, brown the ground beef over medium-high heat. Once cooked, transfer to a bowl and set aside.
Leave the beef fat in the pan, add butter, and melt.
Toss in diced peppers and onions. Cook over medium heat for about 5 minutes until soft.
Meanwhile, boil the pasta shells until tender. Drain and let cool slightly.
Add the beef back to the pan with the veggies. Stir in Worcestershire sauce, ketchup, salt, and pepper.
Stuff each shell with the beef mixture and place in the baking dish.
Tear cheese slices into quarters and place one piece on top of each shell.
In the same pan (now empty), combine milk, beef broth, and cornstarch. Stir over low heat.
Tear up the remaining cheese slices and add to the sauce. Stir until melted and smooth.
Pour sauce over the stuffed shells and bake for 15 minutes, or until cheese is melted and bubbly.
Serve warm and enjoy!