Prep all ingredients as indicated above.
Rinse the herring filets under cold water and pat dry with a paper towel. (If using raw herring, cook to an internal temperature of 145°F (63°C).
Slice the rolls in half and lightly toast them in a toaster oven or oven broiler on low.
In a small bowl, mix together the mayonnaise and Dijon mustard. Season with salt and black pepper to taste.
Spread the mayonnaise–mustard mixture on the bottom halves of the 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 rolls on the sandwiches to complete the broodje haring.
Blend all the ingredients for the mango lassi until smooth. Adjust sweetness to taste. Refrigerate if not serving immediately.
Serve the broodje haring with potato chips, pickle spears, pear slices, and mango lassi.