Have whiting cleaned and skinned. Make a slit down the back of each fish on either side of the backbone to make a pocket. Snip off the bone at each end with scissors and carefully lift it out.
Sauté finely chopped mushrooms, parsley and onion in butter until onion is transparent and cooked through. Stuff each fish with
Transfer poached fish to a heated serving dish; cover with hot Mornay Sauce; sprinkle with freshly grated Gruyère and glaze under a preheated grill until golden and bubbly.