Preheat oven to 350 degrees F (175 degrees C). Place three of the tomatoes, the 1/2 cup olive oil, red wine vinegar, sugar, parsley, mint, basil, oregano, capers, and garlic in the bowl of a food processor and process to create a fresh tomato sauce. Season with salt and black pepper; set aside. Chop the remaining tomato; set aside.
Heat the 2 tablespoons olive oil in a skillet over medium heat, and cook and stir the onions until slightly golden, about 10 minutes.
Stir together the onions, potatoes, eggplant, zucchini, bell peppers, okra, the reserved chopped tomato, and the fresh tomato sauce, and place the mixture in a large baking pan. If needed, stir in a little water so that the vegetables are just covered with sauce.
Bake in the preheated oven until all vegetables are tender, about 1 hour.