Cut the eggplants and zucchinies across into slices about 2cm thick, then arrange them in layers in a colander, sprinkling each layer with salt. Top with a weighted plate and leave to drain for 1 hour
Heat the oil in a large heavy-based pan over low heat, add the onions and fry gently for about 10 minutes until soft but not coloured. Stir in the tomato puree and cook for 3-4 minutes, stirring occasionally.
Rinse eggplant and zucchini in cold water, then pat dry with absorbent paper and add to pan. Stir in the garlic and peppers, cover and simmer for 20 minutes.
Add the tomatoes and the rest of seasoning, stir, and leave to cook for a further 40-45 minutes. Remove the lid for the final few minutes to let the sauce reduce if the dish seems too liquid. Taste and adjust the seasoning before serving.