Preheat oven to 325. Pack tomatoes into a baking dish in a single layer. Sprinkle with salt and sugar. Drizzle 2 tablespoons of oil over. Roast for 1 hour. In frying pan, warm 1/3 cup oil. Add onions and saute until onions are carmelized about 5 minutes. Steam or boil potatoes for 20 minutes. Slice the potatoes and place in a serving bowl. Add remaining oil, onions and both kinds of tomatoes. Season with salt and pepper. Mix and serve.