Preheat the oven to 425 and line a sheet pan with parchment paper.

Slice the eggplant into 1/2 inch thick rounds.

Use a cookie cutter or a knife to cut out a round hole in the eggplant rounds, leaving a small edge.

Set the edges onto the sheet pan spread out, then chop the middle parts into small cubes.

Heat the olive oil over medium-high heat, add the eggplant and the onion and sauté for about 10-12 minutes until tender and cooked.

Add the salt, pepper and spinach and continue to sauté for about 3-4 minutes, until spinach is completely wilted.

Remove from the heat and set aside to cool for about 10 minutes.

Add the mixture to a mixing bowl, then add the feta, mozzarella and Parmesan and mix to combine.

Add the egg and mix to combine.

Add the mixture to the eggplant round edges on the sheet pan, making sure to fill them completely.

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