In a small bowl or jar, whisk together the lemon juice, honey, Dijon mustard, salt, and pepper until well combined. Slowly drizzle in the olive oil while whisking continuously until the dressing is emulsified and smooth.
In a large salad bowl, combine the spring greens, sliced cucumber, blueberries, and chopped pistachios. If using any optional additions like red onion or herbs, add them now.
Drizzle about half of the dressing over the salad ingredients. Using salad tongs or two large spoons, gently toss the salad until everything is lightly coated. Add more dressing to taste, but avoid overdressing.
Sprinkle the crumbled feta cheese over the top of the tossed salad. Give one final very gentle toss to distribute the cheese, or simply leave it on top for presentation.