Prep all ingredients per the instructions above.
Add the cucumber slices to the water. Refrigerate to infuse.
Rinse the salt-cured herring fillets under cold water to remove excess salt, then pat dry with a paper towel.
Slice the rolls lengthwise and lightly toast until just warm and slightly crisp.
In a small bowl, mix the mayonnaise and Dijon mustard. Season with salt and black pepper to taste.
Spread the mayonnaise–mustard mixture evenly inside the rolls.
Layer with onion, sliced pickles, parsley, chives, and herring filets. Cut in half if preferred.
Serve the broodje haring with chips, pickle spears, mango slices, and cucumber water.