Heat oil in large saucepan over medium heat. Sauté onion 3 to 5 minutes, or until translucent. Add zucchini, and cook 8 minutes more, or until vegetables are well browned. Add vegetable broth and 2 cups water, and bring to a boil. Stir in beans and spinach, and return to a boil. Reduce heat to medium-low, and simmer 5 minutes, or until spinach is wilted. Stir in lemon juice, zest, and mint. Season with salt and pepper, if desired.