Slice the cucumber into thin rounds or half-moons about 1/8-inch thick.
Cut the cooked beets into 1/2-inch cubes.
Slice the red onion thinly and soak in cold water for 5-10 minutes to reduce sharpness, then drain.
In a small bowl, whisk together olive oil, lemon juice, salt, and pepper. Add honey and Dijon mustard if using.
In a large serving bowl, gently combine the cucumber slices, beet cubes, and red onion.
Drizzle the dressing over the vegetables and toss gently to coat everything evenly.
Sprinkle the fresh chopped dill over the salad and gently toss once more.
If using feta cheese, add it just before serving.
Let the salad rest for 5-10 minutes before serving to allow the flavors to meld together.
