These make-ahead shells are a perfect party dish you can freeze weeks in advance, and super kid-friendly!
DO-AHEAD: Make and fill the pasta shells
Cook the shells: Fill a large (6- to 8-quart) pot with water and bring to a boil over medium-high heat. Add the salt to the boiling water, then the pasta shells.
Cook according to package directions until the shells are al dente (tender but still firm to the touch), about 8-10 minutes.
Drain and rinse the pasta shells in a large colander, and cool to room temperature.
Make the filling: While the pasta shells cool, mix the filling together: In a large mixing bowl, stir the beef, 1/2 cup ketchup, mustard, relish, Worcestershire sauce, pepper and 1 cup shredded cheddar together with your hands or a large spatula.
Whisk the remaining 1 cup ketchup and tomato sauce together in a separate bowl.
Assemble the shells: You can assemble and bake the shells in any combination of oven-safe dishes (such as 9x13-inch casseroles or 8-to-9-inch square baking dishes), depending on how and when you plan on serving them. If you're freezing the baked shells for future reheating and serving, make sure your dishes are also freezer-safe.
Ladle enough tomato sauce to lightly cover the bottom of the casserole dish(es) you'll be baking and serving the shells in.
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 dishes tightly with foil and refrigerate up to 18 hours.
DINNERTIME: Bake and serve
Bake: Preheat the oven to 400 degrees F.
Bake the shells, covered, for 30 minutes, then uncover and bake for 10 minutes more until the cheese and sauce are bubbling.
Serve or freeze: Serve immediately, or cool to room temperature, then re-cover completely and tightly with foil. Refrigerate baked shells for up to 1 day freeze for up to 1 month. Uncover and reheat in a 350 degree oven until warmed through, about 30-45 minutes for refrigerated shells and 1 hour to 90 minutes for frozen shells.