Preheat oven to 350 degrees F (175 degrees C).
Brush eggplant slices on both sides with olive oil and place them on a baking sheet.
Sprinkle garlic powder and bake for 10 minutes.
Heat 2 tablespoons of olive oil in a large skillet over medium heat.
Add onion, garlic, tomatoes, and spinach.
Cook and stir for a few minutes until fragrant, and until the tomatoes have released their juices.
In a medium bowl, mix together 1/2 cup of ricotta cheese, 1/2 cup of mozzarella cheese, and 1/2 cup of Parmesan cheese. Set aside.
Place the eggplant slices in a greased 9x13 inch baking dish.
Top with spinach mixture.
Spoon the cheese mixture over the spinach and spread into a thin layer.
Pour spaghetti sauce over the cheese layer.
Sprinkle the remaining mozzarella and Parmesan cheese over the top.
Sprinkle with Italian seasoning.
Bake for 30 minutes in the preheated oven or until heated through.
The eggplant can be easily pierced with a fork when baked well.