Bring a large pot of salted water to a boil and cook the jumbo shells until just al dente. Drain and set aside to cool slightly.
Heat the olive oil in a skillet over medium heat. Add the ground beef and cook until browned, breaking it up as it cooks.
Add the diced onion and minced garlic to the skillet. Cook for 3 to 4 minutes until the onion softens.
Stir the squeezed spinach into the beef mixture and cook for another minute. Remove the skillet from the heat.
In a mixing bowl, combine the ricotta, egg, Italian seasoning, salt, and black pepper. Fold in the beef and spinach mixture along with half of the mozzarella and all of the Parmesan.
Spread 1 cup of marinara sauce in the bottom of a baking dish. Stuff each shell with the filling and arrange them in the dish.
Spoon the remaining marinara over the shells and sprinkle with the rest of the mozzarella. Bake at 375°F for 20 to 25 minutes until the cheese is melted and bubbly.