Preheat your oven to 350 degrees.
In a large skillet, brown the Italian sausage, chopping it with a spatula until it is thoroughly cooked and no more pink remains. Drain any grease that accumulates.
As the meat is cooking, boil a pot of water and cook the pasta shells according to package instructions in salted water. Drain the shells, rinse them, and allow them to cool until you can handle the pasta shells.
In a large bowl, stir together the cream cheese and the cooked meat until the cream cheese is thoroughly blended. Add the cottage cheese and the mushrooms, along with the beaten egg. Stir in one cup of the mozzarella cheese and the Parmesan cheese. Add the salt and pepper. Stir all of these ingredients until blended.
Spray a nine inch square baking pan with cooking spray.
In a large bowl, stir together the tomato sauce, onion powder, dried basil, parsley, oregano, and salt.
Pour half of the tomato sauce mixture into the bottom of the pan.
Spoon the filling mixture into the shells and arrange them in the baking pan on top of the pasta sauce in rows.
Pour the rest of the tomato sauce over the top of the shells. Sprinkle the remaining cup of mozzarella cheese over the top of the entire pan of pasta.
Cover the pan with aluminum foil and bake the pan for 30 minutes, covered with foil. Uncover the pan, turn the heat in the oven up to 400 degrees. Return the pan to the oven for another 10 to 15 minutes until the cheese is lightly browned and bubbly.
Remove from the oven and let the dish stand for 10 to 15 minutes before serving.