Preheat grill or grill pan to medium-high heat.
Brush zucchini slices with olive oil and season with salt and pepper.
Grill zucchini for 2–3 minutes per side until tender with grill marks. Allow to cool slightly.
In a small bowl, whisk olive oil, lemon juice, Dijon mustard, garlic, salt, and pepper for dressing.
Arrange mixed greens, cherry tomatoes, cucumber, and red onion in a large bowl or serving platter.
Top with grilled zucchini and grilled chicken if using.
Sprinkle feta cheese over the salad.
Drizzle dressing evenly over salad and toss gently before serving.
Garnish with fresh herbs or avocado if desired.