In a large bowl, whisk together lemon juice and mustard. While whisking, slowly add oil until fully combined. Stir in dill and red onion, then season with ¼ tsp. salt and pepper. Set aside.
In a pot of salted boiling water, cook orzo according to package directions until al dente. Drain.
Add orzo, cucumbers, cherry tomatoes, chickpeas, and olives to dressing and toss to coat. Fold in feta and serve with more dill on top.
