Preheat the oven to 350°F (175°C). Spread about 1 cup of marinara sauce evenly across the bottom of a 9x13-inch baking dish.
Bring a large pot of salted water to a rolling boil. Add the jumbo shells and cook 8–9 minutes, just under al dente so they remain firm enough to handle. Drain in a colander and set aside to cool slightly.
Heat the olive oil in a large skillet over medium-high heat. Add the minced garlic and cook 1–2 minutes until fragrant.
Add the ground beef to the skillet, sprinkle with 1/2 teaspoon salt, and break it up with a spatula. Cook until no longer pink, about 6–8 minutes. Drain excess fat and transfer the cooked beef to a bowl.
In a large mixing bowl, beat the eggs. Add the ricotta, Parmesan, oregano, basil, onion powder, 1/2 teaspoon salt, and 1/2 teaspoon black pepper; stir until fully combined.
Combine the cooked ground beef with the cheese mixture, or layer them separately when filling—either method works to distribute the beef evenly.
Spoon a generous amount of the filling into each cooled shell and place the stuffed shells in rows in the prepared baking dish on top of the marinara sauce.
Dot the tops of the shells with the remaining marinara sauce, then sprinkle the shredded mozzarella evenly over the filled shells. Cover the dish with aluminum foil.
Bake covered for 20–25 minutes, until heated through and the cheese is melted and gooey. Remove foil and serve warm.