In a soup pot, heat the oil until shimmering. Add the onions and cook over moderately high heat until softened, about 5 minutes. Add the garlic and cook for 1 minute. Add the green beans and stir to coat with the oil. Stir in the sun-dried tomatoes, plum tomatoes and water and bring to a boil. Season with salt and pepper, cover and simmer until the beans are tender, about 15 minutes. Add the mint and sugar and cook for 1 minute, then stir in the vinegar. Serve warm or at room temperature.