Bring the water, apple cider vinegar, sugar and mustard seeds to a boil in a saucepan and let them gently simmer until the seeds are soft. Then let the mustard caviar cool and set aside until use.

Meanwhile, finely chop the sweet-and-sour pickles and the parsley. Mix the mayonnaise with half of the parsley and a little pickle juice until smooth to make a dipping sauce.

Deep-fry the frozen bitterballen until brown and crispy, then drain on paper towels. They can also be prepared in an air fryer or oven.

Divide the bitterballen over a serving platter and spoon over some of the mustard caviar, the mayo sauce, the pickle, the sweet-and-sour pearl onions and the remaining parsley. Serve the rest of the mustard caviar on the side. Enjoy!