For the salad: Bring a large pot of lightly salted water to a boil. Add the orzo and cook at a lazy boil until just tender, about 9 minutes. Transfer to a colander and drain well, then transfer to a large serving bowl. Add the tomatoes, cucumber, radishes, olives, cheese, basil, mint and parsley to the orzo.
For the vinaigrette: Whisk together the shallot, mustard and lemon juice in a small bowl. Add the oil in a steady stream, whisking constantly. Season with salt and pepper.
Pour the vinaigrette over the orzo salad and toss well to distribute the ingredients evenly. Serve warm or at room temperature. Or make ahead and chill. Garnish with the pepitas before serving.
Cook's Note: To toast pepitas, lightly coat the seeds with olive oil. Season with kosher salt, then spread in a single layer on a baking sheet. Roast at 400 degrees F, stirring occasionally, until golden and fragrant, 5 to 8 minutes. You'll know they're almost done when you hear them start to pop.