Heat the oven to 375°F. Cook shells in water with the tablespoon salt until 2 minutes short of the package time, then drain gently and separate on a plate.
Brown beef and onion in a skillet for 7 to 8 minutes until the meat reaches 160°F. Drain excess fat and stir in chili powder, cumin and filling salt for 30 seconds.
Add cream cheese and water and stir over low heat until the cheese melts into the beef. Mix salsa and tomato sauce in a separate bowl and spread 1 cup in a 9-by-13-inch baking dish.
Fill shells evenly with the beef mixture and arrange open-side up in the dish. Spoon remaining sauce over and between the shells and scatter cheddar on top.
Cover and bake for 25 minutes, then uncover for 5 to 8 minutes until bubbling and the filling reaches 165°F. Rest for 5 minutes and serve three shells per person.