- First, cook the shells according to instructions on the box but cook them al dente, about 8 to 10 minutes. Drain and rinse the pasta in a colander, and let cool.
- Filling: While the pasta shells cool, mix the filling together in a larger mixing bowl. Combine beef, 1/2 cup ketchup, mustard, onion, relish, Worcestershire sauce, pepper, and 1 cup shredded cheddar together with your clean hands.
- In a separate bowl, whisk the remaining 1 cup ketchup and tomato sauce together.
- Assemble the shells: Ladle enough tomato sauce to lightly cover the bottom of the 9×13 casserole dish.
- With your hands, fill each cooled pasta shell with a handful of filling (about 1/4 cup worth), and place shells filling side up in the baking dish. Continue until all the shells are filled.
- Drizzle the remaining sauce over the shells and sprinkle the remaining cheddar cheese evenly on top.
- Cover the dish tightly with foil. You can either cook it right away or it can be refrigerated overnight. Preheat oven to 400 degrees.
- Bake, covered for 30 minutes, and then uncover and bake for 10 minutes more or until the cheese is bubbling.