Place the onions, lemon juice, salt & pepper in a med bowl. Mix gently to combine. Add anchovy paste, if using.
Cut the baguette in half lengthwise. Place the halves cut-side up on a very large piece of plastic wrap. Drizzle the olive oil and vinegar evenly over both pieces.
Add tuna with its oil to the onion mixture. Stir & mash until well-combined. Add olives & mix well. Add parsley & mix to combine. Spoon the tuna mixture evenly over the bottom piece of bread.
Arrange anchovies evenly over the tuna. Arrange tomato slices over the anchovies. Place egg slices evenly over the tomatoes. Arrange the green beans on top. Top with the other half of the baguette.
Wrap the plastic wrap securely around the sandwich and squash it down with your hands. Wrap securely in aluminum foil. Refrigerate until the sandwich is well-chilled, the bread is soft inside, and the tomatoes have released their juices but the crust is not falling apart, 2 to 4 hours. Cut the sandwich into 4 equal pieces just before serving.