Preheat the oven to 350°F (175°C). Spread about 1 cup of marinara sauce evenly over 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 slightly firm and don’t fall apart. 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, season with 1/2 teaspoon salt, and break it up with a spatula. Cook until no longer pink, about 5–8 minutes. Drain excess grease and set the beef aside to cool slightly.
In a large bowl, beat the eggs with a fork. Add the ricotta, Parmesan, oregano, basil, onion powder, 1/2 teaspoon salt, and 1/2 teaspoon black pepper. Mix until combined.
Stir the cooked ground beef into the cheese mixture (or gently fold beef into the mixture as preferred) until evenly distributed.
Spoon the cheese-and-beef filling into each cooked shell and place the filled shells in rows in the prepared baking dish over the marinara sauce.
Place dollops of the remaining marinara sauce over the tops of the stuffed shells, then sprinkle the shredded mozzarella evenly over everything. Cover the dish with aluminum foil.
Bake for 20–25 minutes, until heated through and the cheese is melted and gooey. Remove foil for the last 5 minutes if you want the cheese slightly browned. Serve warm.