In a large serving bowl, toss together summer squash, red onion, beans, and sun-dried tomatoes. In a small bowl, whisk together olive oil, vinegar, mustard, rosemary, shallot, salt, and pepper. Pour olive-oil mixture over bean-squash mixture and toss until well coated. Refrigerate for at least 30 minutes or several hours to allow flavors to blend. Serve chilled.