Cook the potato in lightly salted boiling water for 10–15 minutes, or until just tender.
Drain and run under cold water until the potatoes are cool enough to handle. Slice each quarter into 3mm thick slices and set aside.
Add the onion to a non-stick frying pan with a little olive oil and cook over medium heat for 5 minutes, or until soft but not coloured. Remove from the heat.
Make a dressing by combining the vinegar, mustard and fromage frais in a bowl and mix well. Add the onion, sliced gherkins and warm potato slices to the bowl and mix until well combined.
Place the Rollmops in a non-stick frying pan with a little olive oil over medium heat, turning so they cook evenly on all sides until lightly browned.
Remove from the pan and chop each Rollmop into 3 pieces, about 2cm wide.
Add the Rollmop pieces to the potato mixture and mix until well combined.
Serving Suggestions: Garnish with fresh parsley and serve with a green salad.