In a medium bowl, whisk together the olive oil, vinegar, lime juice, mustard, salt, and pepper. Add the tomatoes, avocado, scallions, and cilantro and toss gently to combine.
Divide arugula evenly between 4 salad plates. Spoon the tomato-avocado mixture over each serving, dividing evenly, drizzling vinaigrette over the arugula. Serve.