Preheat the Oven: Preheat your oven to 350°F.
Cook the Pasta: Cook the jumbo pasta shells in a large pot of salted water for 2-3 minutes less than the package directions. Drain, rinse with cold water, and set aside.
Sauté the Onion and Garlic: Heat a large skillet over medium heat and add the olive oil. Sauté the chopped onion with ¼ teaspoon of salt until translucent, about 3-5 minutes. Add the chopped garlic and cook for 1 minute until fragrant, stirring frequently.
Cook the Beef: Add the ground beef and ¼ teaspoon salt to the skillet. Sauté until browned and cooked through, breaking it into small pieces, about 5-7 minutes. Drain excess oil if desired.
Add the Marinara Sauce: Stir in the marinara sauce until well combined. Remove from heat.
Prepare the Ricotta Mixture: In a large bowl, mix together the ricotta cheese, egg, thawed and well-squeezed spinach, 1 cup of the Italian cheese blend, ¼ cup of Parmesan cheese, and the remaining ¼ teaspoon salt. Stir to combine.
Fill the Shells: Fill each cooked and cooled shell with about 2 heaping tablespoons of the ricotta-spinach mixture.
Assemble the Dish: Spread half of the beef marinara sauce on the bottom of a large baking dish. Arrange the filled shells on top. Cover with the remaining beef marinara sauce and sprinkle with the remaining 1 cup of Italian cheese blend and ¼ cup of Parmesan cheese.
Bake the Dish: Cover the baking dish with foil and bake for 40 minutes. Uncover and bake for an additional 5 minutes until the cheese is melted and bubbly.
Serve: Sprinkle with chopped fresh parsley if desired, and serve hot.