Prep ingredients: Prepare all ingredients as indicated, slicing the onion, pickles, pears, and herbs before assembling.

Prepare herring: Rinse the herring filets under cold water and pat dry with a paper towel. If using raw herring instead of salt-cured, cook until it reaches an internal temperature of 145°F (63°C).

Toast rolls: Slice the sub rolls in half and lightly toast them in a toaster oven or under a low oven broiler for 1–2 minutes, until just warmed and slightly crisp.

Make sauce: In a small bowl, combine the mayonnaise and Dijon mustard, then season with salt and black pepper to taste.

Assemble sandwiches: Spread the mayonnaise–mustard mixture evenly on the bottom halves of the toasted rolls. Place one herring filet on each roll, then top with sliced onion and pickles. Sprinkle parsley and chives over the toppings and close the rolls.

Make mango lassi: In a blender, blend the frozen mango, yogurt, milk, ground cardamom, and sugar until completely smooth. Adjust sweetness to taste and refrigerate if not serving immediately.

Serve: Serve the broodje haring alongside potato chips, pickle spears, pear slices, and the mango lassi.