Bring a large pot of salted water to a boil. Add the green beans and cook until bright green and tender, 3 to 5 minutes. Drain and transfer to a bowl of ice water. Drain again and return to bowl.
Place the anchovies in a bowl and smash with a fork. Add the vinegar and 2 tablespoons of the oil. Drizzle over the beans and toss; set aside.
In another bowl, combine the tuna, tomatoes, olives, onion, pepper, and remaining oil.
Halve the baguette lengthwise, then cut it crosswise into 4 portions. Toast if desired.
Spoon the tuna mixture onto the bottom of each baguette portion. Sandwich with the top half and press gently but firmly. Serve with the beans.