Preheat the oven to 375 degrees F (190 degrees C).

Fill a large pot with lightly salted water and bring to a rolling boil; stir in pasta shells and return to a boil. Cook uncovered, stirring occasionally, until shells are nearly cooked through but still very firm to the bite, about 5 minutes. Drain well and rinse with cold water.

While shells are cooking, heat a large skillet over medium-high heat; cook and stir beef and onion in the hot skillet until beef is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Stir in tomato paste, tomato sauce, salt, oregano, and garlic powder. Bring to a simmer and cook sauce for 5 minutes. Spread 1/2 of the sauce into a 9x13-inch baking dish.

Beat eggs in a large bowl until smooth. Stir in cottage cheese, 3/4 of the mozzarella, Parmesan, parsley, salt, and pepper until well combined. Spoon cheese mixture into cooked pasta shells and place into the prepared baking dish. Pour remaining sauce over filled shells and sprinkle with remaining mozzarella cheese.

Bake in the preheated oven until shells are hot in the center and cheese is melted and bubbly, 25 to 30 minutes.