Begin by boiling a large pot of salted water. Once boiling, add the jumbo pasta shells and cook until al dente, about 10-12 minutes. Drain and set aside to cool slightly.
In a large skillet over medium heat, add the diced onion and minced garlic. Sauté for about 2-3 minutes until translucent. Then add the ground beef and cook until browned, about 5-7 minutes. Drain excess fat.
Stir in the spinach and cook until wilted, about 2 minutes. Add basil, garlic powder, oregano, salt, and pepper. Mix well and remove from heat.
In a mixing bowl, combine the beef and spinach mixture with half of the mozzarella and parmesan cheese. Stir until evenly incorporated.
Preheat oven to 375°F (190°C). Stuff each cooked shell with a generous amount of filling and place seam-side up in a greased baking dish.
Pour marinara sauce over the stuffed shells, ensuring they are well-coated. Sprinkle remaining mozzarella and parmesan cheese on top.
Cover with aluminum foil and bake for 20 minutes. Remove foil and bake for an additional 10 minutes, until cheese is bubbly and golden.
Remove from oven and let cool for a few minutes. Serve warm, garnished with fresh herbs if desired.
