Peel and finely chop the onion. Peel garlic and chop finely. Wash oregano, shake dry and chop coarsely except for a few leaves for garnishing. Heat the oil in a pot. Sauté onion and garlic in it until translucent.
Add tomato paste and sweat briefly. Add the chopped tomatoes and chopped oregano. Season with salt, pepper and sugar. Simmer for about 10 minutes, stirring occasionally.
In the meantime wash the tomatoes, grate them dry and cut them into slices. Clean and wash the aubergines, cut them into approx. 0.5 cm thick slices and place them on a baking tray lined with baking paper. Drain the mozzarella and pluck.
Season the sauce to taste with salt and pepper and spread on the aubergines. Place the tomatoes on top. Add the mozzarella and bake in the preheated oven (electric cooker: 250 °C/ convection oven: 200 °C/ gas: see manufacturer) for about 10 minutes.
Remove and garnish with oregano.