Rinse the salad greens under cold water and drain well.
In a large bowl, combine the salad greens with cherry tomatoes and cucumber.
Add the thinly sliced red onion to the mixture.
Sprinkle the roasted pistachios over the top.
If using, add the crumbled feta cheese.
In a small bowl, whisk together the olive oil and balsamic vinegar.
Drizzle the dressing over the salad.
Season with salt and pepper to taste.
Toss gently to combine all ingredients.
Garnish with fresh herbs before serving.