For the salad: In a medium bowl, combine the arugula, tomatoes, strawberries, cucumbers, basil, mint, salt and onion. Toss gently to combine. Pile the salad onto a platter.
For the dressing: Using the same bowl, combine the mascarpone, vinegar, Dijon, salt and 2 teaspoons water. Whisk well until smooth. Spoon the creamy dressing over the salad and serve.