Wash and thoroughly dry the mixed greens using a salad spinner or clean kitchen towels.
Halve the cherry tomatoes along their equator (not through the stem end).
Slice the cucumber into thin, even pieces about 1/8-inch thick.
Slice the red onion very thinly. For milder flavor, soak in ice water for 10 minutes, then drain and pat dry.
In a large salad bowl, gently combine the mixed greens, halved tomatoes, cucumber slices, and red onion.
In a small bowl, whisk together the olive oil and balsamic vinegar until emulsified. Add any optional dressing ingredients if using.
Drizzle the dressing evenly over the salad ingredients, using just enough to lightly coat everything.
Season with salt and freshly ground black pepper to taste.
Toss the salad very gently using salad tongs or two large spoons.
Top with crumbled feta cheese and any optional toppings just before serving.
Serve immediately for best flavor and texture.