How to Make It
In a large saucepan of boiling salted water, cook the yellow beans until tender, about 2 minutes. Using a slotted spoon, transfer the beans to a colander and refresh with cold water. Add the cranberry beans to the boiling water and cook over moderate heat until tender, about 15 minutes. Drain the beans, transfer to a large bowl and let cool slightly. Cut the yellow beans into 2-inch lengths and add them to the bowl.
In a small bowl, combine the olive oil, lemon juice and garlic. Add the dressing to the beans along with the tomato and onion. Season with salt and pepper and toss well. Serve slightly chilled or at room temperature.