In a large pot of boiling salted water, blanch the green beans for about 3 minutes until bright green and tender-crisp. Drain and immediately transfer to an ice bath to stop cooking. Once cooled, drain again and set aside.
In a large bowl, combine the salad greens, chopped tomatoes, black olives, flaked tuna, blanched green beans, quartered hard-boiled eggs, and red onion.
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, salt, and pepper until well combined.
Drizzle the dressing over the salad and gently toss to combine, ensuring all ingredients are coated.
Serve immediately, garnished with fresh basil leaves if desired.