Preheat the oven to 425°F (220°C). Line the sheet pan with parchment paper.

Slice the eggplant into 1/2-inch (1.25-cm) thick rounds. Cut a circular center from each round, leaving a narrow eggplant border.

Arrange the hollow eggplant rounds in a single layer on the lined sheet pan. Chop the removed eggplant centers into small cubes.

Heat 3 tablespoons olive oil in the skillet over medium-high heat. Sauté the chopped eggplant and yellow onion for 10–12 minutes until tender.

Add the salt, black pepper, and baby spinach to the skillet. Sauté for 3–4 minutes until the spinach is fully wilted.

Remove the vegetable mixture from the heat. Let it cool for 10 minutes.

Transfer the cooled vegetable mixture to the mixing bowl. Add feta cheese, mozzarella cheese, and Parmesan cheese.

Mix the filling until the cheeses are evenly distributed. Add the egg and mix until fully incorporated.

Spoon the filling into each hollow eggplant round. Press lightly to fill each round completely.

Brush the exposed eggplant edges with additional olive oil. Bake for 25–27 minutes until the tops are golden and the eggplant is tender.

Brush the edges with more olive oil, then place into the oven for 25-27 minutes.