Prep all ingredients as listed above.

Rinse the salt-cured herring filets under cold water to remove excess salt and pat dry with a paper towel; if using raw herring, cook until it reaches an internal temperature of 145°F (63°C) before proceeding.

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

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

Spread the mayonnaise–mustard mixture inside the rolls.

Layer with sliced onion and pickles, herbs, and herring filet. Cut in half if preferred.

Add the lemon slices to the sparkling water, add ice if desired, and stir gently.

Serve the broodje haring alongside potato chips, pickle spears, pear slices, and lemon sparkling water.