In a large pot, heat the oil over medium heat. Add onion, garlic and salt and cook until softened, about 5 minutes. Add the tomato paste and stir to coat the onions. Cook for 2 minutes to caramelize the tomato paste.

Add the eggplant, bell peppers, and tomatoes. Cook for 5 minutes and stir occasionally until vegetables start to soften and release liquid.

Cover the pot and simmer for 20 minutes or until vegetables are very soft, stirring occasionally. If there is no liquid in the pot, add 100ml of water.

Remove from heat and stir in the olives, capers and vinegar. Taste and add more salt or vinegar, if needed.