Prepare the Vegetables
Bring a saucepan of salted water to a boil. Add the potatoes and cook until fork-tender, about 10-12 minutes. Remove and let them cool before slicing. In the same water, blanch the green beans for 2-3 minutes, then transfer to an ice bath to stop cooking. Drain and set aside.
Prepare the Vinaigrette
In a small bowl, whisk together the olive oil, red wine vinegar, Dijon mustard, minced garlic, salt, and pepper until well combined. Taste and adjust seasoning as desired. Set aside.
Assemble the Salad
On a large serving platter or individual plates, layer the lettuce as the base. Arrange the blanched green beans, sliced potatoes, cherry tomatoes, cucumber, olives, and hard-boiled eggs around the platter in sections. Place the flaked tuna in the center or spread evenly over the salad.
Drizzle with Vinaigrette
Pour the vinaigrette over the assembled salad, making sure to distribute it evenly. Use tongs to gently toss if desired, or leave arranged for a traditional presentation.
Garnish and Serve
Garnish the salad with fresh parsley or thyme, if desired. Serve immediately, or chill briefly before serving.