In a large bowl, combine the romaine lettuce, cucumber, cherry tomatoes, bell pepper, shredded carrots, red onion, radishes, and sunflower seeds. Toss gently to mix the vegetables evenly.
In a small bowl, whisk together the olive oil, apple cider vinegar, honey, Dijon mustard, salt, and pepper until well combined.
Drizzle the dressing over the salad and toss to coat all the ingredients evenly.
If using, sprinkle the feta cheese on top of the salad before serving.
Serve immediately or chill in the refrigerator for about 30 minutes to enhance the flavors.