Wash the eggplants, cut the stem ends and bottoms and slice them into ½ cm circles. Arrange them on paper towels, add salt and let them stand for 20 minutes.
Preheat the oven at 220°C. Wipe off the excess water from the eggplants and arrange them on a baking pan lined with parchment paper. Sprinkle oregano on top of them and bake for 25-30 minutes, turning them upside down halfway through. Alternatively, you can grill or broil them.
Meanwhile, prepare the pizza sauce. Purée the tomatoes using a food processor. In a small saucepan, heat the olive oil over medium heat and add the onion. Stir until soft and add water, tomatoes, herbs, salt and pepper. Slightly stir, reduce heat and let it simmer for 20 minutes or until it starts to thicken. Let it cool for 10 minutes.
Prepare your mini pizzas. Spread 1-2 tablespoons of tomato sauce on each eggplant slice. Add the olives, feta cheese and pepper slices. Sprinkle the grated mozzarella cheese and top with the fresh mozzarella slices.
Bake them at 200°C for 15 minutes.