Preheat the oven to 400°F.
In a mixing bowl, combine the spinach, parmesan, bechamel, nutmeg, salt and pepper. Transfer to a baking dish and sprinkle with breadcrumbs and more parmesan. Bake for 12-15 minutes, until golden and bubbly on top.
In a small saucepan, melt the butter for the au poivre sauce over medium-low heat. Add the shallots and a pinch of salt and pepper. Sauté until softened and translucent. Add the sherry and bring to a boil.
Add the beef broth and soy sauce. Bring to a simmer for 15 minutes.
In a small bowl, combine the cornstarch with 1 tablespoon cold water to make a slurry. Stir into the sauce and bring to a boil.
Reduce the heat to low, add the cream and green peppercorns and simmer for 10 minutes. Adjust seasoning with salt and pepper.
Dry the steaks and coat with salt and pepper.
Heat the oil in a large skillet or a grill over high heat. Add the steaks and sear each side for 2 to 3 minutes, until dark brown. Transfer to a baking sheet and finish cooking in the oven to your desired doneness, 8 to 12 minutes. Rest for 5 minutes before slicing on the diagonal.
Divide the spinach gratin and steaks between plates to serve. Drizzle with sauce.