Wash all vegetables thoroughly under cold water.
Tear the mixed greens into bite-sized pieces and place them in a large salad bowl.
Slice the cucumber and add it to the bowl.
Halve the cherry tomatoes and mix them in.
Thinly slice the red onion and add it to the salad.
Sprinkle sunflower seeds over the top.
If using, crumble feta cheese on the salad.
In a separate bowl, whisk together olive oil, apple cider vinegar, salt, and pepper.
Drizzle the dressing over the salad just before serving.
Toss the salad gently to combine all ingredients and serve immediately.