Begin by bringing a large pot of salted water to a boil. Cook the jumbo pasta shells according to package instructions until al dente. Once cooked, drain and set aside on a baking sheet to cool slightly.
In a large skillet over medium heat, add the ground beef and diced onion. Cook until the beef is browned and the onion is translucent, about 5-7 minutes. Drain excess fat if necessary.
Add the minced garlic to the skillet and sauté for 1-2 minutes until fragrant. Then, stir in the spinach, basil, garlic powder, oregano, salt, and pepper. Cook until the spinach wilts, about 2-3 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine the beef and spinach mixture with half of the mozzarella and all of the Parmesan cheese. Stir until well combined.
Preheat your oven to 375°F (190°C). Spread a layer of marinara sauce on the bottom of a baking dish. Using a spoon, fill each cooked pasta shell with the beef and spinach mixture, placing them seam-side up in the dish.
Once all the shells are stuffed, pour the remaining marinara sauce over the top. Sprinkle the remaining mozzarella cheese on top of the sauce.
Cover the baking dish with aluminum foil and bake for 25 minutes. Then, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and slightly golden.
Let the stuffed shells cool for a few minutes before serving. Enjoy your delicious Spinach and Ground Beef Stuffed Shells with a side salad or garlic bread for a complete meal!