In a large pot, soften the bell peppers, onion and garlic in the oil without browning, about 10 minutes. Add the tomato paste and cook for about 2 minutes. Add the broth, cherry tomatoes, tuna and olives. Bring to a boil. Cover and simmer gently for about 5 minutes.
Add the green onions and parsley. Season with salt and pepper.
Serve the soup with cucumber tea sandwiches.
This Niçoise soup also goes well with egg sandwiches. For variety, serve the soup with a poached egg in the center.