Trim and slice the radishes thinly, about ⅛-inch thick. A mandoline works well if available.
Optional: Place sliced radishes in cold water for 5-10 minutes to reduce their peppery bite and increase crispness. Pat dry thoroughly.
In a large bowl, combine radishes, mixed greens, cucumber, and red onion (if using).
In a small bowl, whisk together lemon juice, olive oil, salt, pepper, dill, and honey (if using) until well combined.
Pour the dressing over the salad and toss gently to coat all ingredients evenly.
If using feta cheese and/or toasted nuts/seeds, sprinkle them over the top just before serving.
Serve immediately for maximum freshness, or chill for 15 minutes to enhance crispness.