Heat 1 tablespoon oil in a large saucepan over medium heat. Add onion and cook, stirring occasionally, until tender and light brown, about 5 minutes. Add 1 tablespoon fresh mint (or 1 teaspoon dried), cinnamon, 1/2 teaspoon salt and pepper to taste; cook, stirring, until fragrant, about 30 seconds. Add tomatoes and water and increase heat to high. Cook, stirring occasionally, until the tomatoes begin to break down, 2 to 3 minutes. Stir in green beans. Reduce heat to medium, cover and cook, stirring occasionally, until the green beans are tender, about 12 minutes.