Place beans in a large saucepan; add water to cover. Add 1/2 teaspoon salt. Bring to a boil. Cover, reduce heat to maintain a simmer and cook until the beans are tender, 20-30 minutes.
Meanwhile, cut corn kernels from the cobs: Hold an ear by its stem end in a deep bowl. Use a small sharp knife to cut off the kernels, letting them fall into the bowl. Then scrape down the cob with a small spoon, scraping the "milk" and remaining corn pulp into the bowl.
When the beans are done, drain, reserving the cooking liquid.
Heat oil and butter in a large, heavy skillet over medium heat. Add the corn and "milk." Stir to coat well, then add squash (or zucchini), the beans and 2 tablespoons of the bean cooking liquid. Cook, stirring occasionally, until the corn and squash are tender, 8-12 minutes. Add more bean-cooking liquid if necessary to keep the mixture from sticking to the pan. Season with the remaining 1/4 teaspoon salt and pepper. Sprinkle with scallions and serve immediately.
Serving Sizes:2/3 cup each
Calories:128, Fat:7g, Sat Fat:2g, Mono Fat:4g, Cholesterol:5mg, Carbs:15g, Protein:3g, Fiber:3g, Sodium:300mg, Potassium:330mg