Brush the potatoes under cold water and cook them, with the skin, in a large pot of salted water until tender.
Let them cool completely before peeling and cutting into cubes.
Set aside in a bowl.

Remove the casing from the cervelas and cut it into small pieces. Add to the potatoes.
Finely chop the shallot and cornichons, and chop the parsley, and add to the bowl.

Make the vinaigrette by mixing the mustard and vinegar. Season with salt and pepper, then whisk in the oil in a thin stream (like making mayonnaise).
Add the vinaigrette to the salad, mix well, cover with plastic wrap, and refrigerate.
Before serving, mix again.
To be enjoyed very fresh.