Wash and drain the raw potatoes at least twice to remove all starch. Place the raw and mashed potatoes in a large pot, add the flour and milk. Season and blend to a smooth consistency. Drop a small ladle full onto a hot griddle; push the mixture from the center outwards with the bottom of the ladle, cook for 2 minutes then flip the boxty over and cook through. Season with salt and pepper.
Sear the fillets on both sides in an oiled, well-heated pan, remove from the pan and set aside. Fry the onions and mushrooms until soft, add the whiskey (careful not to burn your eyebrows off), add the cream, cracked pepper and season with salt, to taste. Reduce a little then return the fillets to the pan continue cooking until the sauce thickened to the right consistency.
Place 3 medallions onto each hot boxty, cover with sauce and roll over to make an omelette shape spoon some sauce on the top and garnish with watercress.
This recipe was provided by professional chefs and has been scaled down from a bulk recipe provided by a restaurant. The Food Network Kitchens chefs have not tested this recipe, in the proportions indicated, and therefore, we cannot make any representation as to the results.