In a large pot of salted boiling water, cook the jumbo pasta shells until al dente, about 10-12 minutes. Drain and rinse under cold water to stop the cooking process.
In a mixing bowl, combine the ricotta cheese, half of the pesto, half of the mozzarella cheese, Parmesan cheese, egg, garlic powder, salt, and pepper. Mix until well blended.
Spread a layer of marinara sauce on the bottom of a 9x13-inch baking dish.
Carefully fill each cooked pasta shell with the ricotta mixture, placing them open side up in the baking dish.
Once all shells are filled, drizzle the remaining pesto over the top and sprinkle with the remaining mozzarella cheese.
Pour the rest of the marinara sauce around the shells, covering the bottom of the dish.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden brown.
Allow to cool for a few minutes, then garnish with fresh basil leaves before serving.
Notes
Allow to cool slightly before serving for better flavor.