In a large bowl, combine the prepared greens, tomatoes, cucumber, bell pepper, onion, chickpeas, and sunflower seeds.
Separately, whisk together olive oil, red wine vinegar, Dijon mustard, salt, and pepper in a small bowl. Taste the dressing and adjust seasonings as needed.
Pour the dressing over the salad and toss well to coat. Serve immediately with optional avocado and pepperoncini slices on top.