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.
Meanwhile, place the anchovies in a small bowl and smash them with a fork. Add the vinegar and 2 tablespoons of the oil. Drizzle the anchovy mixture over the green beans and toss; set aside.
In a separate bowl, combine the tuna, tomatoes, olives, onion, pepper, and the remaining oil. Halve the baguette lengthwise, then cut it crosswise into 4 portions. Toast the baguette, if desired. Spoon some of the tuna mixture onto the bottom of each baguette portion. Sandwich with the top half and press gently but firmly. Serve with the green beans.
Tip: If you don't like anchovies, try a large pinch of kosher salt instead.