Make the marinade for tuna. Combine parsley, dill, olive oil, mustard, capers, and lemon juice until well mixed. Add vinaigrette to tuna along with mayo and mix well to combine. Stir in walnuts and season with salt and pepper.
Assemble sandwiches. Top crusty bread with arugula, tuna salad, eggs, shallots, and olives followed by remaining bread. Serve.
Notes
To remove the sharp onion flavor, soak shallots in ice-cold water for 10 minutes before using.This recipe can easily be doubled or tripled.