Cook pasta shells in salted water in a large 6- to 8-quart pot to al dente, according to package directions. Drain water and place shells on a clean dish towel or paper towel to cool.
Meanwhile, cook the ground beef and onion in a large skillet until meat is cooked through. Drain grease, if needed.
Transfer ground beef and onion to a large bowl. Add mustard, worcestershire, pepper, ½ cup ketchup, and 1 cup sharp cheese and mix together.
In a small bowl, whisk together remaining 1 cup of ketchup and tomato sauce. Spoon enough of the sauce mixture to lightly cover the bottom of the baking dish (one 9x13 or two 8- or 9-inch baking dishes).
Fill each pasta shell with a little less than ¼ cup of ground beef filling mixture and place filling-side-up in the baking dish.
Cover shells with remaining sauce, then sprinkle the 2 cups cheese evenly over top.
Cover with foil and bake right away, or refrigerate for up to 18 hours before baking, or follow freezing instructions below.
Bake covered with foil for 30 minutes at 400°F/200°C, then remove foil and bake uncovered for about 5-10 minutes, until cheese is melted and sauce is bubbling.
Garnish with sliced green onion, fresh diced tomato, and/or bacon crumbles, if desired.
Preparing for the Freezer:
Before baking, cover with a layer of plastic wrap, then a layer of foil. Label and freeze. Freeze for up to 1 month.
Prepare After Freezing:
Before baking, remove plastic wrap and re-cover with foil.
Let thaw in the refrigerator, bake as directed above. Or to bake from frozen, bake for about 1 hour and 15 minutes at 350°F/175°C. Then remove the foil and bake uncovered for 15-30 more minutes, until bubbly and warmed through.
Notes
A 12-ounce box of jumbo pasta shells usually includes about 44 shells, some of which may be broken.
This recipe will make 20-24 stuffed pasta shells, depending on how full you fill each shell.