Mashed green beans are a vibrant side dish for all your grilled or roasted meats and fish, and the secret to preserving their bright color is a pinch of baking soda in the cooking water, and then also to immediately rinse the beans under cold water to stop the cooking. After that, they are finished with sauteed garlic, a splash of vinegar, and some flour, cooking off the starchy taste. Add more vinegar to taste before serving.