Remove the steak from the refrigerator 30 minutes before cooking. Using a sharp knife, trim away most of the surface fat. In a small bowl, stir together the vadouvan, salt and a few grinds of pepper. Set aside.
Preheat an oven to 450ºF.
To prepare the creamed spinach, bring a saucepan three-fourths full of water to a boil over high heat. Add the spinach and cook for 30 to 45 seconds. Transfer to a sieve to drain, then rinse the spinach with cold running water and drain again. When the spinach is cool enough to handle, squeeze out as much water as possible with your hands, then coarsely chop the spinach and set aside.
In a fry pan over medium heat, melt the butter. Add the mushrooms and cook, stirring occasionally, until softened, 3 to 4 minutes. Add the shallot and cook, stirring, for 1 to 2 minutes. Add 3 Tbs. of the cream, increase the heat to medium-high and bring to a boil. Cook for 1 to 2 minutes. Add the spinach and season with salt and pepper. Continue to cook until the spinach is heated through, 1 to 2 minutes. Remove from the heat and add the nutmeg, the remaining 1 Tbs. cream and the lemon zest. Cover to keep warm.
Preheat a heavy ovenproof fry pan over high heat until very hot. Pat the steak dry with paper towels. Rub the steak on both sides with the spice mixture. Add the steak to the pan and sear for 2 minutes. Turn the steak over and immediately transfer the pan to the oven. Cook until an instant-read thermometer inserted into the thickest part of the meat registers 130ºF for medium-rare, 7 to 8 minutes.
Transfer the steak to a carving board, cover loosely with aluminum foil and let rest for 5 minutes. Cut the steak into slices 3/8 inch thick and arrange on warmed individual plates. Spoon the creamed spinach alongside and serve immediately. Serves 2.