Blanch the green beans in boiling water about 2 minutes, or until just crisp but tender. Drain, rinse under cold water and pat dry.
In a large bowl, combine the cooked green beans with tomatoes, scallions, olives, capers, garlic, salt, pepper and 4 T parsley. Drizzle in the olive oil, folding ingredients gently. Let rest at room temperature for at least 1 hour for flavors to blend.
To serve, flake the tuna in large pieces; add to the sauce. Add the cooked pasta and toss gently. Adjust seasonings and garnish with remaining T of parsley. Serve at room temperature.