Preheat the oven to 375. Cook the pasta according to the package directions, then drain.
While the pasta is cooking, mix together the ricotta, spinach, egg, parmesan, garlic powder, salt, and pepper, and set aside.
In a large skillet over medium heat, cook the ground beef until browned and cooked through. Drain the ground beef, then add the pasta sauce to the skillet and heat until warmed.
To assemble, spread about 1 cup of the meat sauce mixture in the bottom of a 9 x 13 baking dish. Scoop about two tablespoons of the cheese mixture into each of the shells, and place in the baking dish. Repeat with remaining shells. Spoon the remaining meat sauce over the shells, and sprinkle with the mozzarella cheese.
Bake 30 minutes, until the cheese is melted and bubbly.