Preheat the oven to 400 degrees F (200 degrees C).

Brush cut sides of eggplant with olive oil; place, cut-sides up, onto a baking sheet.

Roast in the preheated oven for 30 minutes, then allow to cool slightly. Keep the oven on.

Meanwhile, brown Italian sausage in a skillet over medium-high heat; drain off grease. Transfer sausage to a mixing bowl; season with garlic powder, Italian seasoning, and black pepper. Stir in 1/2 cup spaghetti sauce, 1/2 cup mozzarella cheese, bread crumbs, and egg; mix well.

Scoop out flesh of roasted eggplant, once cooled, to within 1/2-inch of the skin to create a shell. Roughly chop eggplant flesh; fold into sausage mixture. Divide evenly among two eggplant shells; sprinkle with remaining 1/2 cup mozzarella cheese.

Bake in the 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 1 1/2 cups spaghetti sauce in a saucepan over medium-low heat to serve with the eggplant.