Place the potato strips in a bowl with the lemon juice and season. Press around the salmon fillets or steaks, wrap in a damp cloth and refrigerate for at least a half hour.
Heat the oil and gently cook the fish for 5 minutes or so on each side.
Meanwhile have ready the fennel blended with the butter (again, well chilled) and just before serving put little pieces of this butter into the frying pan in which you have cooked the salmon. Add the stock and blend together. Serve with the salmon.