Bring 8 cups of cold, salted water to a boil in a medium saucepan. Add the orzo and cook until al dente, 7 to 8 minutes. Drain well.
While the orzo cooks, whisk together the mustard, vinegar and some salt and pepper in a medium bowl until smooth. Slowly whisk in the olive oil until emulsified. Set aside.
Combine the orzo, green onions, tomatoes and cucumbers in a large bowl; season with salt and pepper and toss to combine. Scatter the feta and dill over the top, drizzle with the vinaigrette, and toss again. Serve immediately, or chill for up to 8 hours.