If you make this salad ahead of time, the tomatoes will juice and the beans will marinate. This is the way I prefer it- with lots of vinaigrette, sweetened by the tomatoes, and soft, slippery beans. —Food52
Fill a medium saucepan with water. Season generously with salt. Bring to a rolling boil, and add the beans. Cook until tender on the edges but still quite crisp, about 4 minutes. Drain and rinse under cold water. Dry on a towel. Cut the beans in half, then combine in a large serving bowl with the tomatoes.
Whisk together the mustard and vinegar with a pinch of sugar and a pinch of sea salt. Gradually whisk in the oil, letting it thicken and emulsify as you go. Pour the dressing over the beans and tomatoes and toss to mix. It's ok if the dressing pools in the bottom of the bowl. Someone can later mop it up with bread. Season generously with freshly ground black pepper and toss once more. Let sit for at least 15 minutes before serving.