Preheat your oven to 350 F.
Add the ground beef to a large nonstick skillet and cook for 5-6 minutes, until there is no pink left in the meat. Remove the beef and set aside.
To the skillet add the butter, onions and red bell pepper, cooking for 4-5 minutes while stirring to brown and caramelize.
Add the cooked beef back into the skillet. Add the ketchup, Worcestershire sauce, and salt and pepper. Remove from the heat.
Using a small spoon, scoop the beef filling into the cooked pasta shells.
Using the same skillet over medium heat, add 1 tbsp butter and 1 tbsp flour, whisking to create a paste (roux). Slowly add the milk, continuously stirring, to create a thickened sauce. Then slowly add the beef stock until well combined. Add 1.5 cups of the shredded cheese and stir until melted.
Pour half of the cheese sauce into a casserole dish. Place the stuffed shells over the sauce. Sprinkle the remaining 1/2 cup of cheese over the shells.
Bake the stuffed shells for 10 minutes to melt the cheese. Serve hot with remaining cheese sauce and chopped chives.