Preheat oven to 400º F. Line a baking sheet with parchment paper. Set aside.
Cut the eggplant in half lengthwise and scoop out the center, leaving about 1/2 inch around the sides so that it holds its shape when baked. Reserve the scooped eggplant.
Brush the inside of the eggplant shells with 1 tablespoon of oil and roast in the oven, with the inside facing up for 10-15 minutes, or until tender. Set aside.
Heat 2 tablespoons of oil in a skillet over medium heat. Sauté onion for about 3 minutes, or until fragrant. Add garlic and sauté for 1 minute. Add the chopped reserved eggplant and cook until tender, about 5-8 minutes. Add 1 1/2 cup of the marinara sauce, season with oregano, salt, and pepper to taste and cook for about 5 minutes.
Spread the remaining marinara sauce over the bottom of a baking dish.
Mix panko and Parmesan in a small bowl.
Place the eggplant shells in a baking dish; fill them with the sauce, top with panko mixture and mozzarella.
Bake in the preheated 400º F oven for 10-15 minutes, or until the cheese has melted and the sauce is bubbling.
Garnish with fresh basil and serve.