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Preheat the oven to 350 degrees.
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Add the ground beef to a large cast iron skillet (this browns very well) and brown until a deep brown crust appears before breaking the beef apart.
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Stir the ground beef and brown until a deep crust appears on about 50 or so percent of the beef.
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Remove the beef (you can leave the fat) and add the butter and the onions and bell peppers.
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Let brown for 1-2 minutes before stirring, then let brown for another 1-2 minutes before stirring again.
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Add the beef back into the pan.
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Add the ketchup, Worcestershire sauce, salt and black pepper into the pan and stir.
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Take it off the heat and scoop it into the pasta shells.
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Top each shell with cubes of cheese (use half the cheese for this).
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Using the same pan add the beef broth, milk and cornstarch and whisk before turning the heat back on.
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Add in the rest of the cheddar cheese a little at a time while whisking for 3-5 minutes or until thickened.
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Pour about half the sauce around the shells.
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Bake in the oven for 10 minutes to melt the cheese.
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Serve with the remaining sauce.