Combine all of the SANDWICH FILLING ingredients in a prep bowl. (Or use a mini-chopper or food processor and PULSE into small pieces.) Taste and adjust if needed. Note: I prepared the sandwich filling minus the pistachios the night before I planned to serve it. When I was ready to prepare the sandwiches the pistachio nuts were stirred in then.
Just prior to serving, divide and spoon the sandwich filling on half of the bread slices. Top with other bread half and cut into quarters.
Arrange on a platter and garnish with fennel fronds. Do not prepare the sandwich too far in advance or they will become soggy.