Preheat the oven to 350°F (175°C).
In a large pot, bring salted water to a boil. Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a skillet over medium heat, add a drizzle of olive oil. Sauté the minced garlic for about 1 minute until fragrant. Add the ground beef, breaking it apart with a spoon. Season with salt, pepper, and Italian seasoning. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat.
In a mixing bowl, combine the cooked ground beef, ricotta cheese, half of the mozzarella cheese, grated Parmesan cheese, and the egg. Mix until well combined.
Pour half of the marinara sauce into the bottom of the baking dish. Stuff each pasta shell with the beef and ricotta mixture and place them seam-side up in the baking dish.
Once all shells are filled, pour the remaining marinara sauce over the top. Sprinkle the remaining mozzarella cheese over the sauce.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it cool for a few minutes. Garnish with fresh basil if desired, and serve warm.