Start by making the sauce. Place the parsley, fromage frais, cream cheese, stock and capers in a food processor and blend until smooth. Season well, transfer to a bowl, and set aside until ready to serve.
Preheat the grill until hot. Place the salmon fillets on a rack in a single layer and season well. Grill for 8-10 minutes, or until the fish has cooked through.
Place each fillet on a warmed plate, spoon over the sauce, and serve with asparagus, if wished.