Cut eggplant in half lengthwise, and cut around insides of each half leaving a 1/4-inch thick border. Scoop out the flesh with a spoon and coarsely chop.
Brush insides of eggplant shells with oil and season with with salt and pepper. Place cut-side-up on a baking sheet. Roast @350F until tender, about 20 minutes.
Pour marinara sauce into a baking dish. Transfer eggplant shells to dish. Spoon filling into shells, top with the remaining mozzarella, and then the breadcrumbs.