First, make the sauce. Put a pan of salted water on to boil. Push in all the watercress and boil for 5 minutes. Add the spinach and cook for another minute or so until wilted. Drain in a colander. Press with the back of a ladle to extract as much moisture as possible.
Put the leaves in a food processor and blend to a fine puree, scraping down the sides occasionally. Pour 1/4 cup of the cream through the processor funnel and keep the blades whirling for what seems like an eternity. You will eventually get a sauce with a texture like silk. It will be so smooth, you will not need to pass it through a sieve. If you want a pouring sauce, add the remaining cream. Check the seasoning, and pour into a small saucepan for reheating.
Cut the beef loin into 4 even steaks and rub each side using 1 tablespoon oil. Heat a heavy, non-stick frying pan until you can feel a good heat rising. Lay in your steaks-they should give a good hiss as they hit the hot pan. Season the tops and cook for about 3 minutes, then flip over and cook the other side for 2 minutes. Slip the butter into the pan at this stage and season the second side. Remove the steaks from the pan and leave to rest while you cook the mushrooms.
Saute the mushrooms with the garlic in the remaining oil. Season and mix with the parsley.
Reheat the watercress puree. Place the steaks on warmed plates (sliced first if you like), trickle any pan juices over, and spoon on the mushrooms. If your watercress puree is firm, shape it into quenelles. If it is a sauce, spoon some over the steaks and pass the rest separately. Serve whatever accompaniment you like to eat steak with. Fries are great, or why not try slices of really fresh baguette?
Recipe courtesy of Gordon Ramsay's A Chef for All Seasons