For the berenjenas gratinadas, cut the eggplants into slices. Put the slices in water with salt and vinegar and let them steep for about 10 minutes. In a blender, puree the tomatoes with the garlic and onion and heat in a pot or pan until the flavor develops.
Season to taste with pepper and salt. Pat the eggplant slices and place them in a baking dish, spread the tomato sauce and cheese over them. Bake in the oven at about 200°C (convection oven 175°C) until the eggplants are soft and the cheese has melted.