Fill a large pot about half full of water. Add 1/2 tsp of salt if desired. Cover and heat over high heat until the water is boiling rapidly. Add pasta shells. Heat to boiling again. Boil uncovered 11 to 13 min, stirring frequently, until tender. While the water is heating and the pasta shells are cooking, continue.
Peel and chop onion. Cook groud beef and onion in a skillet over medium heat until beef is brown, drain any excess fat.
Stir the chili powder, cream cheese, and 1/4 cup of the taco sauce into the beef in skillet. Heat over medium low heat 2-3 min, stirring occasionally, until cheese is melted. Remove from heat.
Heat oven to 350'. Spray casserole or baking dish with cooking spray.
Drain the pasta shells in a strainer. Fill the shells with beef mixture. About 2 tablespoons. Place filled shells in pan. Pour rest of 1/2 cup taco sauce over the shells.
Cover with aluminium foil and bake 20 minute Remove the pan from oven. Sprinkle cheese (and corn chips if desired) over shells. Bake uncovered about 10 min longer or until cheese is melted. Peel and slice the green onions. Garnish pasta shells with sour cream and green onions, if desired,.