Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally, until tender yet firm to the bite, about 12 minutes. Drain and set aside.

Preheat an outdoor grill for medium heat and lightly oil the grate.

Mix 1/3 cup olive oil and garlic powder in a small bowl. Brush both sides of eggplant slices with oil mixture.

Grill eggplant slices on the preheated grill until soft and lightly browned, flipping halfway through, 8 to 10 minutes total. Let cool.

Warm 1 tablespoon olive oil in a skillet over medium heat. Add onion and cook until soft, about 5 minutes. Add minced garlic and cook until aromatic, 1 to 2 minutes more. Add spaghetti sauce and diced tomatoes; mix well. Add black olives and mix.

Cut cooled grilled eggplant slices into fourths and add to the sauce in the skillet. Mix until coated and warmed through, about 5 minutes.

Add sliced fresh mozzarella cheese just minutes before serving to allow cheese to melt. Serve sauce on spaghetti noodles and sprinkle with fresh basil.