In a bowl mix together the shallots, mustard, and vinegar to make a smooth marinade. Add oil to the required consistency.
Peel the potatoes and chop into wedges. Cook in salted water.
Add in chopped tarragon and parsley. Add the cooked potatoes to the marinade and leave for two hours to marinate.
Heat the oil in a pan and fry the char, initially skin side down.
The fish may then be turned. Add a dollop of butter, a sprig of tarragon and finish on a low heat.