Heat the olive oil in a large skillet or saucepan over medium heat.
Add the chopped onion and garlic and sauté until softened and translucent, about 5 minutes.
Add the diced red and green bell peppers to the skillet and cook for another 5-7 minutes until they begin to soften.
Stir in the diced zucchini and eggplant, cooking for an additional 5 minutes until the vegetables start to soften.
Add the diced tomatoes (or canned tomatoes) to the skillet, along with the sugar. Mix well and bring to a simmer.
Reduce the heat to low and let the mixture simmer for about 20-30 minutes, stirring occasionally, until all the vegetables are tender and the flavors are well combined.
Season with salt and pepper to taste.
If the mixture is too thick, you can add a little water to reach the desired consistency.
Garnish with freshly chopped parsley before serving.
Pisto can be served hot or at room temperature, often accompanied by crusty bread or as a side dish to meat or fish.
Enjoy your meal!