Prep all ingredients as indicated above.

Rinse the herring filets under cold water and pat them dry with a paper towel.

If using raw herring, cook to an internal temperature of 145°F (63°C) before assembling.

Slice the sub rolls in half and lightly toast them in a toaster oven or under an oven broiler on low until golden.

In a small bowl, mix together the mayonnaise and Dijon mustard. Season with salt and black pepper to taste.

Spread the mayonnaise and mustard mixture evenly on the bottom halves of the toasted rolls.

Place one herring filet on each roll. Top with sliced onions and pickles, then sprinkle with fresh parsley and chives.

Place the top halves of the rolls over the fillings to complete the broodje haring sandwiches.

To make the mango lassi, combine the mango, yogurt, milk, cardamom, and sugar in a blender. Blend until smooth and creamy. Adjust sweetness to taste, and refrigerate if not served immediately.

Serve the broodje haring sandwiches with potato chips, pickle spears, apple slices, and chilled mango lassi.