Brush cut sides of eggplant with olive oil and place, cut-side up onto a baking sheet. Roast in preheated oven for 30 minutes, then remove and allow to cool slightly.
Meanwhile, brown the Italian sausage in a skillet over medium-high heat; drain off the grease. Place into a mixing bowl, and season with garlic powder, Italian seasoning, and pepper. Stir in bread crumbs, 1/2 cup of spaghetti sauce, 1/2 cup of mozzarella cheese, and the beaten egg; mix well.
Once the roasted eggplant has cooled enough to handle, scoop out the flesh to within 1/2-inch of the skin to create a shell. Roughly chop the eggplant meat, and fold into the sausage mixture. Divide evenly among the two eggplant shells, and sprinkle with remaining mozzarella cheese.
Bake in preheated oven until the filling has set, and the cheese is bubbly and golden-brown, about 30 minutes. While the eggplant is baking, warm the remaining spaghetti sauce in a saucepan over medium-low heat to serve with the eggplant.