Prep the ingredients per the instructions above.
Preheat the oven to 350β (176β).
Place the cucumber slices in the water. Refrigerate.
Rinse the herring filets under cold water and pat them dry with a paper towel.
Place the fish on a foil lined baking sheet. Season with salt and pepper. Bake in the oven until firm and cooked through, about 6 to 8 minutes. Set aside to cool.
Slice the bread rolls in half and lightly toast in a toaster oven or oven broiler on low.
In a small bowl, mix together the mayonnaise and Dijon mustard. Season with salt and pepper to taste.
Spread the mayonnaise and mustard mixture on the bottom halves of the bread rolls.
Place a herring filet on each roll and top with sliced onions and pickles.
Sprinkle fresh parsley and chives over the toppings.
Place the top halves of the bread rolls on the sandwiches to complete the broodje haring.
Serve the sandwich with carrot sticks, hummus, and cucumber water.